LOCK=NONE not working when adding column

I'm trying to add a column to a large table:

ALTER TABLE `users` ADD `seen_count` int, algorithm=INPLACE, lock=NONE

When I run this, other threads selecting a single row from this table (users) are stalled; "show processlist" indicates "Waiting for table metadata lock"

My understanding was "algorithm=INPLACE, lock=NONE" would not lock up other threads. Or, if lock=NONE wasn't supported (though I believe it should be fine here), an error would occur.