I'm having this problem as well. More specifically, to qualify "free trials" better, I'd like to declare in SKAN conversion values the event "has started the trial but not cancelled within 20minutes" (or send both events separately, doesn't matter). The problem being that those cancellations events aren't recorded by the app itself but third party tools which sync though S2S.
I wonder if there's a way to send back that event to the app, and have its conversion value processed there, but it seems quite unreliable. If you find a way keep us posted!