Bit of chat about it over here too.
The gist is that the Zigbee 3.0 chip from SiLabs used in the C8 appears to not be quite so backwards-compatible as it promises on the spec sheet, which seems to have broken pairing for Iris V1 devices.
Luckily mine mostly remained on my C7, but in my tests I had exactly the same pairing issue as you did. The Iris V1 devices will work and even continue to route traffic on the C8, but if they ever drop the re-pairing never completes.
This may be fixed by SiLabs in a firmware update one day... or maybe not. So keep your C7 and earlier hubs around for the V1 devices and use Hub Mesh, which does perform brilliantly.
