I am not sure what data watchtowers would be privy to regarding these individual transactions be it with a hodl invoice, pre-image or a PTLC, so I'm not sure how they would find out if a async payment failed or wasn't passed on the case of a trampoline relay

From my understanding and messing about watch towers only check channel states to see if both parties have an agreed on final state.

Hopefully, I can hit up a few people for answers and turn it into a post because its a good topic