CompactionManager sometimes wrongly determines that a background compaction is running for a particular table

Sometimes after writing different rows to a table, then doing a blocking flush, if you alter the compaction strategy, then run background compaction and wait for it to finish, CompactionManager may decide that there's an ongoing compaction for that same table.
This may happen even though logs don't indicate that to be the case (compaction may still be running for system_schema tables).