Even with "On Demand" hot water, an open shower faucet with a closed wand can play havoc with water temperature.
With the push button on the wand closed and the valves open, cold water can cross over and dilute what hot water you are getting. This will give you hot water initially, but lukewarm to cold water as the hot water in the pipe is used up.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW