I have seen mention of an issue with Roon ARC Ipv6 support whereby ipv6 ip addresses with (non-zero) quads with leading zeros (explicit or implied) caused issues. e.g. a ‘quad’ of ‘9ab’ (which has an implied leading 0 and so is equivatent to ‘09ab’ as one of the quads would cause issues.
Last I read, Roon were working to fix this but I don’t know the current status.
Edit: It looks like the ipv6 leading zero issue was not universal but restricted to one ISP: