CardDAV group sync
I opened this issue on RCMCardDAV GitHub repo thinking it was a bug of them, while it turned out it was just ignorance (on my side).
TL;DR:
To make a long story short, if you want to sync groups of contacts, using a CardDAV address book, make sure that every client/sync application is using the same format to store groups. This applies to e.g. roundcube, with the plugin of this post's link, DAVx^5 sync app, and generally each client which supports CardDAV.
- rcmcarddav settings in roundcube
- DAVx^5 settings (it's in Italian, sorry. The difference between group types is the same as above though)
Before setting DAVx^5 to the correct option, groups created on roundcube won't sync correctly to my phone, while the other way around worked fine.