jl.quema has stated the switch does not have that issue when paired to the Lyric hub with the same light bulbs. I would absolutely be asking this question as well but if I'm taking what has been said at face value then it can't be the bulbs.
Just to confirm again, the bulbs have not been changed, correct, @jl.quema ?
It is attached to LED bulbs.
When I try on regular incandescent light, there is no flicker.
That was my very first test as I suspected the LED to be responsible for the flickering.
But, since the dimmer works fine with these same LEDs when it is paired with the Lyric, I discarded the bulbs as being the root cause...
But if it was the bulbs, they would flicker when power was low. Correct me if I'm wrong, but don't you have the flickering when the level is too high? I.e. If the level is OVER 50, it flickers.
If the LEDs are bad enough they can flicker when they are low but they might also flicker again in the middle and be okay at full brightness.
So, if for whatever reason they are retaining parameters from Lyric or getting parameters incorrectly when leaving Lyric or joining HE it could still be the issue.
For example, for whatever reason on Lyric they are dimming only from level 25 to 50 and the bulbs handle that. So, it appears to work on Lyric.
But on HE they are not getting a parameter to stop at 50 so they are going from 1 to 99 and that's why he sees a flicker (I'm guessing momentarily at 55) but that goes away at full.
Some LEDs dim sooooo poorly that you can't tell the difference between 50 and 100/99.
That's why I've focused on the parameters. IF at any time the behavior was the same in Lyric as HE (or becomes the same in Lyric and HE) then it is the bulbs. In general, it is ALWAYS the bulbs. DC dimming done controlled by an AC dimmer is just not an easy thing to do and every company does it poorly.
that's correct: the flicker is when the power is above 55% so the opposite of the "regular" led issue
I see what you mean.
To totally discard the LED being the root cause, I put another dimmer (Zooz) in place of the GE and the flicker didn't occur.
I might have missed something but I think that with all my tests I have isolated the issue to be with the dimmer and the hub it is paired with.
You tried a new switch (Zooz) with the old bulb, but did you try the old switch (GE) with a different bulb? Some dimmers are just better than others at dimming. They use different methods of control, filtering circuitry, and most likely differing frequencies of the pulse width modulation. If you have a bulb that can dim at say 400 Hz, yet flickers at 120 Hz, you could be getting some weird interaction between switch, bulb, and so on.
Edit: not to discount what you are seeing, but it seems like there would widespread reports of this issue on this forum IF it were the hub, or having this particular switch on this hub.
To answer your questions, yes:
If the flickering was happening all the time, I would be quick to blame the GE dimmer. But since it happens only when paired with the HE, I'm just confused.
I agree that there would be widespread reports if it were the hub so it has to be the combination: GE dimmer + HE + LED bulbs
I just cannot figure how the hub would play a part given there are so few parameters on the dimmer. Unfortunately, when paired with the Lyric, I have no way to read the parameter values.
No, it can't be because I too have the same combo and experience no flickering. And many, MANY other people have GE dimmer on HE with LED bulbs and this is the very first report I have ever seen of a bulb flickering when the dimmer is too high and stopping when the dimmer is lower. It is common to have bulbs flicker when they are lower than the bulb supports.
When you say flickering, how fast a flicker are we talking? If you need help estimating, you can go heere: http://moodlight.org There you can set the change to strobe and move the slider left & right. it would be helpful if you're talking like a sub 10 Hz or we're talking like >50 Hz.
The flickering is not always at the same rate, the frequency increases with the brightness.
Some values to illustrate:
So, as the current increases, there is no flidering....then at some point, it starts flickering, like the current was too low. The only way to make an LED flash is to cut the current too low. And you have factory reset the dimmer?
When you swap in an incadescent bulb and increase brightness from 1% to 99%, at 55% does the brightness suddenly drop very low and then start to increase again? Or does it brighten smoothly from 1-99%?
You read my mind 
No: the increase decrease on the incandescent bulbs is very smooth, no jump, no sudden drop.
Regarding factory reset: I didn't find how to do it, the docs I found only describe the exclusion/inclusion as a reset method.
You still haven't pulled parameters from the switch while it was connected to the other hub.
Then the bulbs should not be flickering. The incandescent bulb tells us that current is raising steadily the whole way. The LED bulbs should not begin flickering when the current is too high. LED bulbs just don't do that. Have you tried with different bulbs?
And more specifically, another brand/type of LED. I asked for this above, but maybe I wasn't clear that I was looking for a different LED. Although the incandescent results are interesting...
Right! If we'd seen a sudden drop in brightness on the incandescent then I'd understand the flickering. But this just makes me more confused.
Sorry for the silence these two last days.
So...
I was about to give up and removed (excluded) the dimmer from hubitat, paired it again with Lyric and.... this time it flickered with Lyric too (when above 50%)
This is the first time it happens with Lyric (despites at least 50 pair/unpair)
I then decided to put it back in hubitat waiting to replace it with a zooz... and... it doesn't flicker anymore.
Don't ask why, I have absolutely no idea. It does not make any sense to me.
I did the inclusion/exclusion at least 50 times and it was always consistent: flicker with HE, no flicked with Lyric.
Why suddenly a change ? not a clue.
I used again the zwave basic tool, no change there either....
Well, at least it is consistently inconsistent, eh? 
It it had been me, I would have simply replaced the LED bulb with another brand until I found one that behaved. Smart dimmer switches + LED bulbs are not the best of friends, I have found.
How could I read these parameters when it's connected to another hub ?
No, because when I use a dimmer from another brand (Zooz) or when the dimmer is paird with another hub (Lyric), it works... so why blame the LED, especially when it doesn't happen at low voltage but the opposite.
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