Hi,

I have have two bathrooms, the first one gets lukewarm water in both shower and sink ( thus bathroom is the closest to the hot water heater). The second bathroom get full hot water in the shower and sink. Both bathrooms have mixers in shower and sink.
I have checked for a temp reg valve but if it was installed it would be near the hot water service.
I have replaced the sink mixer in the first bathroom and no change, the copper pipe from the wall is not even hot.
Both bathrooms are on the second floor.
Is it common to have a temp reg valve on just the one shower and sink?
Even the plumber is stumped.

No, it is not common to have a temperature regulating valve on just one bath group.

You replaced one sink mixer, now shut off the shut off valves under the other sink mixer and see if hot water shows up in the shower that only has warm water. Give some time for hot water to arrive.

If shutting the valves off fixes the shower the problem is that sink mixer. If shutting the valves off did not fix the issue then the problem is in one of the shower mixing valve cartridges. If this is the case, replace both shower cartridges only and problem should go away.

Mark

If that does not work consider installing a tankless water heater by the bathroom that gives lukewarm water. Its cheap, convenient, and an easy fix to your problem.