Hi Andrew,
Here is another consideration. The knobs on the faucet are supposed to be open in order to temper the water flow at the sprayer. Here is the issue: If the knobs are both left open, it allows the cold water to mix with the hot water and you get lukewarm water to the shower and vanity. You are not aware that the faucet knobs were left open because there is no water flow unless the sprayer is used. Perhaps someone has re-plumbed the output to your sprayer? I had a customer change the thermostat on his water heater which he thought was the problem but did not solve the lukewarm water in the shower. He finally drove 250 miles to see me and all I did was open his water bay door and turn off the knobs to the sprayer faucet and instant hot water in the shower!
Hope this helps, Fred