Hello @JiriZyka,
From the docs:
LOCK TABLES FOR BACKUPuses a new MDL lock type to block updates to non-transactional tables and DDL statements for all tables.
What you are experiencing is correct behavior. You should not be running any DDL during an SST. The SST process needs a consistent schema to create a clone of the node, thus any/all DDL are blocked during the SST process.
Because you ran a DDL, it got blocked, and this block also blocks writes to the table. If you do not run a DDL, then writes will not get blocked.