It's very clear from the slow logs and binary logs that on the master a transaction deletes many rows in multiple tables (up to 1k rows or more) and after that a second transaction deletes only one row.
I suspect that on the "slave" as it fails only when wsrep_slave_threads > 1, the second transaction is executed and committed faster.
I don't know if the writeset issue with FK is related of the amount of rows or tables involved.
Just an extra comment:
It's very clear from the slow logs and binary logs that on the master a transaction deletes many rows in multiple tables (up to 1k rows or more) and after that a second transaction deletes only one row.
I suspect that on the "slave" as it fails only when wsrep_slave_threads > 1, the second transaction is executed and committed faster.
I don't know if the writeset issue with FK is related of the amount of rows or tables involved.