That is interesting that tech support hasn't seen it yet, but again, not surprising since it is fairly new and Pro itself, especially as CU, is still just starting to take off. We only have advanced PRO-CU and I have not tried de-authorizing....but I'm sure going to wait until this is resolved.
I wonder if you can talk to your customer service rep to have that Basic seat re-set so you can try authorizing it again? Or do you think it is an actual LM issue? In either case, seems like a bug to me, and I would ask tech support to elevate the ticket.