The setting innodb_lock_schedule_algorithm=VATS that was introduced
in MDEV-11039 (commit 021212b5)
causes conflicting exclusive locks to be incorrectly granted to
two transactions. Specifically, in lock_rec_insert_by_trx_age()
the predicate !lock_rec_has_to_wait_in_queue(in_lock) would hold even
though an active transaction is already holding an exclusive lock.
This was observed between two DELETE of the same clustered index record.
The HASH_DELETE invocation in lock_rec_enqueue_waiting() may be related.

Due to lack of progress in diagnosing the problem, we will remove the
option. The unsafe option was enabled by default between
commit 0c15d1a6 (MariaDB 10.2.3)
and the parent of
commit 1cc1d042 (MariaDB 10.2.17, 10.3.9),
and it was deprecated in
commit 295e2d50 (MariaDB 10.2.34).