If you’re able to edit the text box (double clicking it will put you in edit mode), the delete works by removing characters ahead of the cursor. However, by selecting multiple boxes you probably exited edit mode which is why delete works as expected.
Thank you for bringing this to my attention. I think some other people are also having this problem. I’ll have to see if I can make it more obvious which mode you’re in and if you’re able to actually delete a text box or not.