Is it obligatory to make up the prayers and perform ghusl after fifty days postpartum when brown discharge reappears, considering that the bleeding had stopped for a period and then resumed with symptoms similar to menstruation?

The blood that appears after forty days from childbirth is istihada (non-menstrual vaginal bleeding), unless it coincides with the timing of the menstrual period, in which case it is hayd (menstrual blood). The blood that appears 21 days after the cessation of the previous blood is hayd (menstrual blood), for which you should abandon fasting, prayer, and sexual intercourse. As for the brownish discharge (kudra), if it occurs during the period of your regular menses, then it is hayd; otherwise, it is not. You must make up the prayers you missed thinking they were not obligatory upon you.