My experience parallels yours.
I've done zigbee channel changes twice on different coordinators. Once on Hubitat (from 18 to 15), and the second time on zigbee2mqtt (from 15 to 25). On both occasions, some devices followed the coordinator. But an equal number needed to be reset and rejoined.
I sent the hub UID in my support request, but am happy to send you MAC address via DM.
Please do, while you and I are around in the community 
This is my guess. I believe that Zigbee channel 11 is what is reported when Zigbee crashes (from unfortunate personal experience), but isn’t actually a channel change.
A bad device might be spamming the hub to the point that the radio can’t join devices or communicate with anything. I once had a Peanut plug send 76,000 events in a few hours on a SmartThings hub. That hub’s Zigbee devices wouldn’t respond and I couldn’t add new devices until I figured it out.
Try unplugging or otherwise cutting power to your repeaters and reboot. Then see if you can add devices before slowly powering up repeaters.
Did you get my DM?
Geez I hope he meant while he was active (online) at the moment. Otherwise:
I did, was checking on your hub. We'll have to dive deeper as I am not sure what's killing your Zigbee. It doesn't look like a hardware malfunction.
Didn't he tell you? He just got hired at Insteon.... He starts monday.. 
Spoken like an extremely experienced support expert. 
The two most important buttons on a computer. The other is:
Download the Hubitat app
