The artist has to be a new name to the db; this does not work if there is at least 1 artist with the given name already in the db.
In two different tabs, have two instances of the RE going. In each, use an entirely new artist (one who would not conflict) for a track artist. In each, proceed to the "missing entities" tab. Add the sortname for the new artist, in each tab. Now proceed to finish out the add release edit in each tab.
You now have two artists with the same name, neither with a comment. Likely, there should be only one, or at worst, 2 but with at least one having a comment.
The data seems to be sanitized to check for such an instance only when the missing entity tab itself is loaded; there is no check when the overall edit is submitted, to see if any of the new entities have been created in that time between the missing entity tab being loaded and the overall edit being submitted.
I've not tested, but I would think this would have the same effect in inadvertently creating duplicate label entries via that same entity tab.