It doesn't say "gain the card from the trash" so it can't lose track of it.
This doesn't follow. Lose-track applies whenever a card tries to move a card that isn't where it's expected to be. ActionThief would expect Fortress to be in the trash, it wouldn't be there, so it wouldn't get gained.
why does ActionThief expect Fortress to be in the trash? All it knows is the card was Trashed.
...Because if you put a card in the trash, that's where you expect it to be?
That's all totally different stuff than my original question, but well...
I second this. If you discard or gain a card, it is supposed to be in your discard pile. That's why you can shuffle Inn back in your deck. So if you trash a card, you expect it to be in the trash.
Hrm. So it turns out on Iso you can't buy an Inn, trigger the shuffle, then use the Royal Seal to fish the Inn out of the deck or discard pile and topdeck it. Which seems a little weird. There is still no assumption the gained card is going to the discard pile (I mean, you can trash a NC with a WT, right?)
But then, I guess you can't reveal a WT, trash a Rats, draw a card, reveal another WT, move the gained card from the Trash to the top of the Deck, reveal WT again, Trash the card again etc... so there is the implication that WT can only move the gained card from the mythical 'the card you are in the process of gaining' spot. Once it actually moves, (i.e. becomes trashed, goes to your discard pile, Inn gets reshuffled, NC topdecks...) WT and Royal Seal loses track and can't move it again, right? So if buying cards also has this hidden location assumption, I guess Thief can too as there aren't any cases that move a card from location A to B or location C to B without clearly specifying "A or C" (like Hermit does).