InnoDB defragmentation periodically hangs when replicated

Periodically, InnoDB defragmentation (innodb-defragment = 1) hangs on the slave in a replicated setup.

It seems to happen only when parallel replication is enabled (specifically slave_parallel_threads = 4 and slave_parallel_mode = 'optimistic').

When the hang occurs, one of the slave workers executing the OPTIMIZE TABLE) statement remains in state "Waiting for table metadata lock" seemingly forever.

Stopping the slave does not appear to work, although manually killing the other slave workers does result in the STOP SLAVE statement to eventually succeed.