Well, I spoke a bit too soon. Today I attempted to connect one more switch. I had problems with that switch earlier but I had thought I just had not tried hard enough to reset it. Today I tried really hard!!
I believe I hit the issue described in this thread. No matter (no pun intended 
) what I did including all the methods described at this reply by @jvm33 , the switch would not pair. I plan to RMA it next. (I had one more VTM31-SN switch left, it paired immediately with no issues so the project was done.)
With method #3 I did find an Inovelli switch listed in the iOS keychain matter accessories that was not any of the successfully paired switches. I removed, it, I retried all the methods etc still no luck. So there is something there…
For the record: the gateway is an Apple TV (A2843) running tvOS 18.2 I assume that the VTM31-SN at the time of the attempt was running firmware 1.0.0 (like the rest of the switches in the batch).