I’m writing this partly to help anyone who experiences it, and partly so I can remember what I did to solve 3 months of frustration.
I have ST with over 125 Matter over Thread devices running happily - that had been rock solid for over a year. Then one day, all 125 of them drop offline and automations for the remaining Zigbee devices stopped working too. Reboot hub. Reboot my home network switches, routers, etc. No luck. Do it all again multiple times. Check my firewall rules. Add explicit IPv6 configuration that wasn’t needed before. No luck. House is dark, literally. ST Support cases go unanswered.
Then suddenly things start working. Falsely thinking it was a firewall rule change, a few days/weeks later it happened again - over 100 devices show as offline that had been working for months. Another ST case goes unanswered.
I noticed the ST v3 Hub is showing an error condition when plugged into my managed switch. Change ports - it works. I thought it might be a failing switch, so replaced that with a known working spare. Same result. ST Hub throws errors that can be remedied with changing ports (sometimes immediately and sometimes after several tries). Suddenly things come up. Maybe it was a bad port.
2 weeks later: 121 Matter over Thread devices again suddenly “fall offline” for a day, then mysteriously 84 come back online overnight, leaving 37 (and most automations) not working… No matter what I do, cannot get these 37 back online for over a week. My wife is incensed and starts demanding “normal” lights and switches.
Last night I came across a thread from 2024 titled “ST +Matter + Hue = Problems” that caught my eye while searching these posts. The actual thread wasn’t related but it made me think - I have disconnected the Hue Hub a time or two from the network during troubleshooting, but never thought it might be a culprit and never really focused on that as a possible cause. Unplugged the hue hub from power for over an hour last night and immediately the ST Matter devices started coming on line. Literally immediately. Within a couple hours, all but 8 devices were back online of the 125. Power cycles brought the last few devcies back.
So a conflict, an error, a hiccup - or SOMETHING must be ogoing on at the Hue Hub that affected the ST Hub in a dramatic way. I am not bashing Hue - in fact, I have always been quite happy with the smooth, instant response it has given, but it just happened to be the device with the gremlin this time.
Hopefully that helps someone in the future!