The implementation of MDEV-1980 is incorrect. The GTID is not synced within the Galera cluster, therefore the gtid_slave_pos cannot be used for reconnecting the replication stream.
This implementation reads to inconsistent slave and not working point in time recovery.
The way to do this is to read the binlog that was retrieved from the old node, find the 'Xid' that belongs to the most recent transaction, then connect to a next node and find the same 'Xid' on the next node's binlog.
The procedure is risky because at SST all binlogs are removed on the joiner node.