Does ablution become void and prayer become invalid if esophageal reflux occurs, reaching near the throat and its taste is felt during the final Tashahhud, and is re-performance obligatory if one forgets the فرض (prescribed prayer) in which the reflux occurred?

If the regurgitated food does not reach the mouth, the prayer is not invalidated. However, if it reaches the mouth and one intentionally swallows it, then the prayer is invalidated, because it is considered akin to eating. If one is obligated to make up a forgotten prayer, but does not know which specific prayer it was, then one must make up five prayers to ensure the discharge of one's obligation with certainty.