Does Wudu become void or is Salah invalidated by a part of the underwear entering the anal opening during Salah?

If you feel a part of your clothing entering the anus during prayer, continue your prayer, and there is no blame on you, and your prayer is valid, whether you are certain that a part of your clothing has entered or you have doubts about it. Prayer is not invalidated except by the occurrence of some of its known invalidators.