Mroonga in wrapper mode over InnoDB works nice under MariaDB Galera cluster except SST transfer where result depend on method:
mariabackup -> corrupted
rsync -> corrupted
mysqldump -> works but just as regular InnoDB tables

Corrupted tables could be repaired with innodb_force_recovery=1 and will be just regular InnoDB tables, also with mysqldump method mroonga FUNCTIONS and INSTALL PLUGIN mroonga SONAME 'ha_mroonga.so' it`s not replicated to joining node.
Using mariabackup/rsync looks like only missing is <db_name>.mrn files from datadir, when you try manual SST of Galera Cluster Node With Mariabackup and just copy this files everything is working aka Mariabackup is not backuping this mroonga related files.

With mysqldump method i try to install mroonga engine before joining but this going end up overwritten.

This is more feature request than bug i presume should not be hard to implement support for mroonga in wrapper mode in Mariabackup and also MariaDB Galera Cluster.