Quick fix that worked for me. Factory reset the S31 & unplug it. Rebuild zigbee network. Plug the s31 in start zigbee pairing mode. I'm not using the C8 for zigbee anymore so wanted to see if I could replicate your problem. Soon as I turned on the C8 it starting pulling Zigbee devices from my C5 fun fun lol. Had to reset the zigbee radio on the C8 to get it to to stop doing that. After that tried to pair a S31 and I would get the same problem you're describing. I rebuilt the c8's Zigbee network and was able to pair the S31's like normal.
OH don't reset your zigbee network on the C8!! that's just something I had to do to get the C8 to stop taking devices off my c5. Hopefully that's not part of the fix for you guys.
