Yes!
I have also had issues with groups (categories / distribution lists). I have been in touch with Philippe about this and specifically about Fastmail and he promised last Friday to take a look after the weekend. He's a bit laconic but very responsive. Probably won't do any harm if you email him at
cardbook.thunderbird@gmail.com and say you also have issues and look forward to a resolution.
The problem I had was that on the first sync with Fastmail I got attributes grouped into sets of categories (it's not worth rehearsing here; I have shared the details with him). My feeling, though I have not read the CardDAV 3.0 spec and I could be wrong, is that it looks close to working. I want to set up a test account and do a few experiments, starting with empty lists.
I'd be interested to know what client if any Fastmail say works best and also what back-end Philippe thinks works best.
Anyway, it it indeed great to get shot of SoGo. Cardbook is improving all the time.