Moen single knob mixing valves tend to fail this way. AIUI, the
cartridge develops a leak from the cold side to the hot. I'm told
a bad Moen valve anywhere in the house can do this to the entire
system. I have one that needs replacement, but other more pressing
things first.

They have a lifetime warranty so if it's a Moen you might try
calling them.

I have no idea what brand it is. There are no markings on the outside
that provide any clues. I'm sure it was the cheapest valve available
when the contractor built this house six years ago.

What's weird is that it's very intermittent. I can have very hot water
for many days and then have one or two days where it doesn't get much
past warm. Very strange.