As an aside, I’ve been watching bcachefs with some interest, as it seems to be getting faster with every kernel release, building on the data integrity guarantees of ZFS while pushing performance boundaries and being GPL compatible (i.e. in tree). Kent Overstreet et al. have done a fantastic job with this FS.
ACLs on Linux can be a bit weird. If I remember correctly, the ACL mask corresponds to the group bit when using masks. Some more details here:
https://unix.stackexchange.com/questions/65888/setfacl-incorrectly-changes-group-permissions