In reply to message posted by Bruce T:
That would have been my first guess SYLVAN!
if the offending shower gets cold real fast yet other fixtures further away are not, it sounds like they might have a re-circ line tee’d off that fixture!
a bad check valve at the re-circ pump will allow cold water to back feed and mix with the hot at its closest point…the tee of the hot water in the wall of the master bath shower!
Thank you, The best ones are the jobs where some P&H fraud connects a branch to a fixture from the return circulation line and then you get a call to find the problem