First off I did have ZigBee signal issues but they are all cleared up now with addition of external antenna.
Remaining issue though is that my sengled color bulbs seem to be "forgotten" when the hub reboots (updating or just regular reboot).
They go unresponsive until I unscrew them for a couple seconds then screw them back in.
These things used to be so nice but now it seems like they are becoming more and more of a hassle.
I'd start with re-pairing them in place without deleting.
You mean running some kind of ZigBee repair? Didn't know that could be commented
I think he means doing a “Pair” again (re-pairing). There isn’t a repair/fix that I am aware of.
With zigbee, when a device is paired again, it will take back the slot it previously had.
I have one Sengled color bulb that seems to do the same thing, for no apparent reason. Pairing it again seems to work, but it stops working again after a bit. It can be annoying…. 
This is exactly why I replaced my Sengleds with Innrs.
This is what I was having to do with my IKEA's that were doing the same thing. Usually, all I had to do was run Zigbee discovery again and they would pop up as "Previously Paired" devices. Then they would work again.
So, the Innr's are better about this? I have been looking at them. However, an independent review I saw said they were flaky after about a year of use (They were specifically talking about integration with the Hue hub. So, it could be just the Hue Platform).
I've had the Innrs in place for about 3 months, and I have them paired directly to HE. There have been no issues for me since they were put into use (hope that doesn't jinx me 
). I have a couple of color bulbs and 6 regulars.
I've been running Sengled for over a year now and haven't seen any issue like this, and don't remember other reports like this for Sengled.
I would do the re-pair as noted, and then do a re-boot of your hub, and if they disconnect again I would suspect mesh problems. Does every Sengled bulb disconnect after a reboot, or just some of them? How many do you have?
I have 3 colors 2 two different styles of white. It's not super consistent about it usually only two RGB and sometimes one white.
Forgive me since I just remember this but to repair I need to factory reset bulb and delete it in devices list. Then just do a new ZigBee discovery, which the bulb will be automatically ready for since being reset.
Just go through the process to reset it and rejoin it to your hub. You don't have to delete it from anywhere.
This and sorry, I replied in a rush last night. I would start by resetting and rejoining the bulbs that give you a fit. Don't make any changes on the hub aside from going back through the "add device" option. Zigbee will recognize it as a previously paired device, but will go back through the backend association process.
Update: I repaired both of the most suspect bulbs. Sunday mornings at 4 am the hub auto reboots. Again they are both unresponsive.
Do they show up in the Zigbee routing table?
Should not need to do this...do you have a specific issue you are trying to solve? These days I never reboot my hub except for FW updates.
Obviously you should not lose control over the bulbs when you do reboot. But if I had an issue that made me need to reboot weekly, I'd want to fix that.
I would suspect your mesh. I have had eight color Sengleds and a few of the whites in service for approximately two years. None of them have ever dropped off. I use a couple of SmartThings smart plugs as repeaters and scattering of generic powered zigbee smart plugs to control some assorted devices.
This is mostly a left over tactic from a couple years ago that I miss guided started to do.
So I'd kill the reboot, and then look at your Zigbee mesh...you may need a repeater or two between your bulbs and the hub.
I'm not a Zigbee master (mine has always worked well so I've never really had problems to troubleshoot) but you want:
LQI >200, hopefully near 255
Incost/Outcost as close to 1 as possible
Devices in your neighbor table who aren't doing well on those numbers may not be great repeaters.
In your routing table see which device are routing through which routers, that could tell some tales.
Something you could do that's no cost/simple, is shut down your hub for 25 minutes. That will put all your Zigbee devices into panic mode and they will re-route when your hub restarts. That may result in better routing choices. You can take before/after screenshots of the child and route info to see what changed, if anything.
Others like @aaiyar who are better w/Zigbee issues may have more to add.
Which bulbs are giving you fits?
Download the Hubitat app
