The BTRFS still has some not solved issues in the mainstream. Almost all of them seems fixed in a Synology’s version of BTRFS. Unfortunately they didn’t share their fix with Community.
I think Synology’s fix is simply to not use BTRFS’s native raid implementation, but instead use just the BTRFS file system on top of mdraid and LVM. Although I guess this issue is probably an issue with the file system itself.