I was wondering about this situation in reverse

I have an XXX-LAX-LHR-AMS return from an ex-EU ticket starting in AMS - will the AA check in desk at XXX want to tag my bag all the way to AMS or can I request them to only tag to LHR?

Given there is only a 90 minute turnaround between my arrival at LHR and the flight to AMS (which I don't want to take) there's a high % chance of me actually missing this flight anyway (as the LAX - LHR is often late) but if my bag is tagged to AMS what will happen at LHR if I miss the connection?