Bathtime puzzler

You need to thing about how hot water tanks work. When the hot is run off from the top of the cylinder, cold water is added to the bottom. What can happen is the cold at higher flow rates starts to agitate the water in the tank so mixing with it. The hot / cold boundary layer goes away and you end up drawing off the hot/cold mix.

I suspect, if you leave the tank to settle for a while, the hot/cold layers might reform and you draw off hot again.