Thanks to Vagrant and Lamby for looking into this. See results of research at: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=985821 https://github.com/mroonga/mroonga/issues/298
I am leaving the issue open as it is not yet fully resolved, but the affecting part is only in Debian unstable builds and for Debian testing/stable builds Mroonga in MariaDB does build in a reproducible way. That together with RocksDB fixed in #976985 makes MariaDB now fully reproducible in Debian Bullseye.
