Water damage or bad patch job on the ceiling directly under the shower stall of master bath?

It is both. Probably.

A section of drywall was removed to fix a leak.

The leak has returned. Now moisture has migrated into the seams of the resectioned area causing discoloration.

Or it could be that a section of drywall was left out during install to leave access to the underside of a shower that had not yet been hooked up.
This was done in my house when the plumber was on vacation and the drywall installation had to proceed.

To be certain that it is active leak use a moisture meter to determine that this area is significantly higher in moisture content than other areas in the ceiling.

Obviously a similar cut out will need to be made so that whatever is leaking( most likely the drain) can be fixed. Then a new patch can be made.

Followed with a stain blocking primer and repaint the ceiling.