i believe its either:

the payer's node is trying to pay the htlc but it gets paid by a different node ( unlikely).

the payer's node is offline and the htlc expires without the payer's node being online. this is the more likely instance as wallets like blixt, mutiny, zeus node in the phone wallets go offline at times.

both will cause a force close and not just for the sending and receiving nodes but can also fuck routing nodes causing their channels to close too. this is why there are people that believe zaplocker is harmful to the network ( and i agree).