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 deprecate the
option and issue a warning that using it may corrupt data. The unsafe
option was enabled between
commit 0c15d1a6 (MariaDB 10.2.3)
and the parent of
commit 1cc1d042 (MariaDB 10.2.17, 10.3.9).