Is it obligatory to repeat the prayer if a slight brownish discharge is found after the prayer, and it is doubted whether it is from the remnants of ghusl and istinja' or if it descended during the prayer, knowing that I am experiencing استحاضة (bleeding outside of menstruation) and blood does not descend during prayer?

If there is a possibility of something occurring in one of two timeframes, it is attributed to the closer of the two. Thus, if there is doubt whether a drop (of impurity) exited before the prayer or during it, its exit is presumed to have occurred during the prayer, and the prayer must be repeated. And if its exit is presumed to have occurred during the prayer or after it, its exit is presumed to have occurred after the prayer, and the prayer is valid.