btrbk is sending full backup with pre-existing backups about btrbk HOT 3 CLOSED

As I can see, there are no march snapshots of rtfs in /mnt/subvolroot/.snapshots/. This means an incremental btrfs send/receive is not possible anymore, since there is "No common parent subvolume present" (see your btrbk log).
btrfs always needs an anchor on both sides for incremental backup, since "btrfs send -p" does not know anything about the target.

I have a question though: Did btrbk delete the march snapshots, or did you do it by hand? btrbk should never delete snapshots if backup failed (which is true if btrbk could not find the target volume)

from btrbk.

Okay, I understand now.

I only delete snapshots when I need to transfer them to the backup drive to prevent the creation of currentdate_1snapshots. However, I only delete the current day and nothing before.

systemctl -u btrbk showed that it didn't delete any snapshots, so it was probably another program on my end. I'll do a closer inspection now.

from btrbk.

Upon closer inspection, I found a program that does bi-weekly defragments to blame. Since btrfs is unable to do snapshot-aware defrags, it compromised by wiping out my snapshots.

I'll go submit a bug report on their end (if this "is" is a bug).
Sorry about that, another hit and miss.

from btrbk.