When MDEV-19514 removed the change buffer merges in the background,
it introduced a minor regression: After a DDL operation that drops
or rebuilds a table or partition, any entries in the change buffer
could survive until a slow shutdown (innodb_fast_shutdown=0) is executed.

fil_delete_tablespace(). Before returning, invoke
ibuf_delete_for_discarded_space(). With this, the only way to have
change buffer ‘leaks’ is to kill and restart the server at an
appropriate moment and never initiate a slow shutdown.