What was this other system? I'm curious because, as far as I know, no distribution officially supports the use of "set-default-subvolume".
I have tested it again. Setting default subvolume is not relevant in this case. This is debian, but settting default is what I used to do in the past.
Yes, reflink means that the data contained in subvC will be nonexclusive to subvC; however, the *subvolume* subvC (and thus subvC_ro) has no relationship to subvolumes A and B.
...
This looks like user error to me, because when clones are used with a single parent, a btrfs stream is sent and recieved;
... You subvolumes A and B were not in a relationship with subvolume C, so this was "user error". Please close the bug. Since 2019 I have learned Btrfs better, sorry for taking your time.
