Before MDEV-15158, wsrep xid information was stored in only one place:
in the TRX_SYS page. Starting with 10.3, it is not stored there but
in the rollback segment header pages, and the latest one is what
matters. MDEV-19229 allows the undo tablespaces to be rebuilt when
innodb_undo_tablespaces is changed on startup. Previously it was not
possible to change that parameter.

These changes caused the fact that rollback segment header pages could
contain several wsrep xid's stored and when undo tablespaces were
rebuilt there was a effort to restore wsrep xid back to rollback
segment header page but because there was several of them the latest
wsrep xid was overwritten with older one.

trx_rseg_read_wsrep_checkpoint
trx_rseg_init_wsrep_xid
	Return true if read xid is wsrep xid, false if not

trx_rseg_mem_restore
	Try to read wsrep xid and if it is found copy it to
	trx_sys.recovered_wsrep_xid if read xid has larger
	seqno.