Yeah they didn't claim the throne I checked that right away but forgot to state that in the first post there was a time where I was maintaining 100 prestige (as was Austria) and they still had the PU going in their favor. I am not in a regency but I do have a queen though I don't think that really effects anything as Aragon frequently has a queen as well.
Hmm, maybe it's the Queen then? I wasn't aware that queens couldn't become a PU-master, but it's certainly plausible.
Given we've ruled out Prestige and Claim the Throne, I think that that has to be the explanation.
By the way, are you able to Claim their Throne? Is that option enabled for you under Aragon's Diplomacy? If Queen's can't be PU master, then I'd expect to see that button disabled, or not even there. Can you check?
Edit: I zoomed way into your picture and can see Claim Throne in the background. It looks like it's enabled, though it's difficult to be sure through the tooltip overlay..
If this isn't Ironman and/or you don't mind experimenting, it would be interesting to see what happens when you click that. It will trash relations and lose you Prestige, though, so you probably don't want to unless you can reload.