Follow-up: It seems that I could solve my problem. Maybe this can also help you know the root cause and reproduce it some how. My buggy btrfs was initially on a single partition, I've freed another partition later and added as `btrfs device add` as the free partition was to the left of the original one so a resize was not possible. Originally I'd metadata as dup and data as single. Even after adding the new device it remained like that for few days, yesterday, I noticed it reported both dup and RAID1 of metadata which was weird. It did some sort of metadata balance on its own when it got a new partition, actually I didn't run such balance command before as both partition are on the same disk, so thought that raid-1 is useless. So I've taken a backup, then run `btrfs balance start -mconvert=raid1 /home/` .. So only raid1 is reported now on `btrfs filesystem df` and `btrfs filesystem usage`. I then run fstrim and it worked fine.
