I have a tub/shower combo in the master bath. The water is HOT when turned on and comes out of the faucet. When I pull the shower tab and turn the shower on, the water is cold. I've let the tub water run a long time and the temperature is always HOT. When I turn the shower off and the water is coming out of the faucet again, its again HOT. All of the facuets in the house are fine. The other shower is fine. This is the only fixture that has this problem. I've changed the shower head and no difference. The water pressure is fine just the temperature issue.
Sounds similar to my "shower only" stall.
A plumber told me there is a set screw under the cover that adjusts the hot/cold mix. He said that many people turn this down when selling a house so that the inspector won't find scolding hot water on inspection. Then new owners can turn it back up for a hot shower.
It did help my situation.
~ Just a thought.
For anyone interested the solution to this problem was me flushing my tankless heater via this post: