Issue:
Mariadb acquires additional MDL locks on UPDATE/INSERT/DELETE statements on table with foreign keys. For example, table t1 references t2, an UPDATE to t1 will MDL lock t2 in addition to t1.
A replica may deliver an ALTER t1 and UPDATE t2 concurrently for applying. Then the UPDATE may acquire MDL lock for t1, followed by a conflict when the ALTER attempts to MDL lock on t1. Causing a BF-BF conflict.

Solution:
Additional keys for the referenced/foreign table needs to be added to avoid potential MDL conflicts with concurrent update and DDLs.