What hub are you using? I was having the same issues on a C4 (like the OP) and eventually the radio just died. The Z-wave radio is fine. I migrated the devices to a C7 and am still working on the Z-wave devices.
C7.
The problem cleared up when I altered some variables in the 700 series driver options for this Zooz dimmer. It's a matter of it being far more capable than earlier units, so, if you turn something on that needs to be off, or if it's the default (no, I have no idea what I changed to stop the problem) it apparently causes issues.
Now, for the next HE issue ... why does my Zigbee radio just decide to turn off for a few minutes once or twice a day? The log says:
| zigbeeOn | Zigbee radio is on | zigbeeOn | SYSTEM | 2021-12-09 12:59:38.570 EST |
| zigbeeOff | Zigbee radio is off | zigbeeOff | SYSTEM | 2021-12-09 12:54:03.537 EST |
Do you have any bulbs in your mesh? If so what brand(s)?
Oh sorry, I didn't catch the context switch from Zigbee to Z-wave. 
The Zwave mesh has no bulbs participating, if that's the question.
On Zigbee my presumption is that all the 22 listed devices are 'home run' connects to the HE C7 hub. They're certainly within RF range. I'm uncertain there is a simple tool to see the Zigbee mesh.
There's a route table you can look at, it's in the manual
zigbee tool ... got it, thanks.
A while ago (Nov 2020) it was mentioned that Zigbee radios can overheat. You have to check with the staff to see if they can see an issue I think.. Anyway here's part of the old thread.. My issue was the one after a reboot and a firmware update fixed it if I recall.. the overheating thing is different.
That's interesting. The C4 that had the Zigbee radio die was sitting on top of my router for a while. When I removed it from service I noticed that the router was running a little warm. Not bad but it was definitely putting out some heat that may have been negatively affecting the Zigbee.
Oh well, it doesn't matter now. I moved the Zigbee devices to a C7 and will be migrating the Z-wave over time. The UI performance on the C4 was always a bit slow. I might end up using it for special projects but it will be good to get it out of the rotation.
Yeah but my post was referring to a C-7 to which I assumed Victor's response below it was related.. could have been mistaken but thought I'd mention it here anyway. 
My C-4's seemed to have higher instances of heat and memory issues.. but that was also me having to learn the eccentricities of my setup..
