mariabackup reports wrong GTID position in presence of replication filters

When a master is also receiving updates from an upstream master having a separate domain ID, and replication filters are in use, all GTIDs from upstream are received and reflected in gtid_slave_pos, but only GTIDs of statements that pass the filters are written to binary logs - so far, so good.

Problems arise when a backup of such master is taken, because at end of backup, the position saved for replication purposes in _xtrabackup_binlog_info _ contains a GTID from the upstream master that is not necessarily one that has passed the filters and was hence binlogged.

As a consequence, when trying to restore such backup and setting up replication, it fails with error 1236:

Got fatal error 1236 from master when reading data from binary log: 'The binlog on the master is missing the GTID 27000-27019-1662000168 requested by the slave (even though both a prior and a subsequent sequence number does exist), and GTID strict mode is enabled'

Of course, that GTID is not in master binlogs, for the reason explained above; replication cannot be set up using GTIDs.