The donor node gets locked up during SST

Hi there,
Recently moved to percona xtradb cluster 5.6 but currently operating on standalone mode due to some issues with the cluster.
It seems the donor node gets blocked when a second node tries to join the cluster and full SST happens.
Using the mysqlcheck script on port 9200 from haproxy, 503 Service Unavailable is being thrown during this period. When SST is over, then the cluster services normally.
I understood the donor should be operable during this time from the documentation.
Can someone clarify if this is a bug or expected bahaviour please?

What method are you using for SST ?