Thermister stops reading temp after a few seconds. (Marlin 1.1.8, Tronxy X5s stock Melzi board)
Thermal expansion is opening a connection somewhere. It might not be one of your solderings, but if you shorted the board bad enough to fry pins, it could have cooked something somewhere else, and putting current through it is heating up the spot enough to break the connection. Either somewhere else in/on the board, or inside of the IC chip itself.