First, when you run the WH above 120-degrees, you must install a tempering valve for safety reasons.
IF the valves are anti-scald, they also have an adjustable limit stop in them to set the maximum shower water outlet. You may need to adjust those. In the process of turning the water off, then filling up the new WH, you could have dislodged some crud in the pipes that is now caught in the valve or cartridges or showerhead. It also would momentarily create a mismatch in the pressure on the hot/cold side of the valve, and if it is an anti-scald valve, it could have jammed the spool valve to one end. Some valves also have check valves inside them, and those can get lodged to one end or obstructed with crud.
Some valves can develop an internal cross-over, and thus mix some hot with the cold all the time. This can limit how hot a local branch can become, since it is already being mixed. A vanity faucet, or a tub/shower one can do this and usually is fixed by replacing the cartridge.
This should give you some things to check, but may not answer your question.