Thanks Tobias for looking into it. When you do testing, please don't do any pinning or other stuff that interferes with apt. There is no need to recompile anything either. The versions of MariaDB and Galera in Debian Sid, Testing and Bullseye are all compiled with the exact same toolchain and are still identical.
Simply test a normal upgrade from Buster to Debian Testing and see if the issue still repeats with all the MariaDB and Galera packages in Debian Testing. - Otto
