As the default button for locking is Alt+M1, this drops the item stack and locks, rather than just locking. I'd prefer if locking slots didn't just disable sorting for those items, but also disallowed dropping so that the above case would not result in the user sometimes dropping their items into the void in the end.
https://github.com/lucaargolo/slotlock has a very good implementation of this, and it might be a good starting spot for a similar feature.