Was the screen cracked? I assume you paid for a new screen replacement and now it's having issues?
If the motherboard is having issues I don't think they should charge you extra for the screen.
They should cover their own screw ups and it's their fault.
Who knows but maybe they knocked off small little components right above the battery connector and in result stops the phone from recognising the battery properly causing intermittent restarting for no apparent reason even on charge.
Or It could be long screw damage (damage to logic board circuit traces) from putting a longer screw than it should of for the metal plate for screen ribbon connectors that has the four screw holes (Two are on the board and those ones are prone to long screw damage).
What repair did you bring the phone in for? (Nearly forgot to ask that)