Hi
Let me start by saying that the behaviour you are experience is expected by design. On 5.7, the xtrabackup command used for SST includes the parameter --lock-ddl (https://github.com/percona/percona-xtradb-cluster/blob/5.7/scripts/wsrep_sst_xtrabackup-v2.sh#L1582) which will execute the LOCK TABLES FOR BACKUP command (https://www.percona.com/doc/percona-xtrabackup/LATEST/xtrabackup_bin/xbk_option_reference.html#cmdoption-lock-ddl)
All this is to guarantee consistency of the backup.
The fix is to avoid running DDLs on the cluster during the SST process (or at least at the beginning of it where all the backup lock happens)