I installed a Bosch tankless hot water heater about 2 years ago. It never did a great job, but at least the hot water was the same throughout the house. Now the hot water in the tub faucet is luke warm at best while the sink in that bathroom gets plenty hot. There is no hot water going to the washing machine in the basement. Other sinks throughout the house are OK. I have all the settings on the Bosch to max for the hottest water possible.

Is this the same winter and summer? How cold are your winters?

Has the tub valve Posi-Temp or Anti-Skald on it? What's the brand name of the faucet? Take the washers hot water hose off at the shut off valve and check and clean the screen. Good luck, Tom