So, above is also a toilet I can say.

Looks like the piping is no more original because I can see more recent PVC pipe just run on the wall.

Probably the piping covered by the ceiling has leak. The leak can easily seeps through the wall and affect the ceiling in Master bedroom which I assume next to this toilet.

This is no about leaking roof because it is ground floor. You just need to get a plumber to locate the leak. Repairing damage ceiling is second step. Need to fix the leaking pipe first.

I'm in Penang which I think different from yours.