Mixing valve
If the wall is wet only when the shower runs, look at the valve.
A failed cartridge or body can dump water into the wall cavity while the shower still ‘works’. The downstairs ceiling becomes the first alarm.
Stop using that shower.
Every extra rinse wets someone else’s insulation.
The handle trim is not the waterproofing.
A pretty plate can hide a wet valve body. Do not keep tightening the handle as a repair.
Both bathrooms should be available on the same visit.
Upstairs valve, downstairs stain. Separate appointments waste the pattern.
Questions people ask
Can I replace the cartridge myself?
Only if the building allows that work and the wall is not already wet. A wet cavity still needs a source record.
Is this the same as a shower-pan leak?
No. A pan leak often follows standing water. A valve leak follows the mixing of hot and cold. Write which.