YoMa, welcome to House Repair Talk. Chimneys leaks are sometimes hard to diagnose. Flashing is one issue. Improperly installed or reusing old flashing can and will sometimes be a problem. When I hear the roofer finally fixed the leaky flashing, it usually means they patched it till it leaks again. A proper chimney flashing should not leak from the start.
Now thats not the only reason a chimney will leak. Water absorption in the mortar and brick, from the sides and top of the chimney, will also show signs of leaking in the same area as flashing. Tuck pointing and sealing of the chimney may be required. I wouldn't fix the drywall till you are positive the water issue is resolved.