innodb_compression_level variable used to specify the level of ZLIB compression to use for InnoDB compressed tables and indexes. It is not persistently in tablespace flags.
Mariabackup fails to retrieve the value from the running server. So it may fail to use the correct compression level while preparing the backup.
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patch is in bb-10.1-thiru (https://github.com/MariaDB/server/commits/bb-10.1-thiru)