This is a user-script bug.
The "Purge all" button and the way it updates the UI is different from how the moderator menu does it.
When the moderator menu purges comments, it follows it with a call to
StackExchange.moderator.loadAllComments($post) which is mostly just a wrapper around
StackExchange.comments.loadAll($post, '?includeDeleted=true'). This displays all comments on the post in their new deleted state.
The user-script is likely just hiding the comments
display:none and the script that shows the add form is interacting with that in an unexpected way. If the script truly wants to remove those from the UI, the best way would be to call
StackExchange.comments.loadAll($post) after purging the comments.