Tankless water heater

I've got a new master bathroom that is being supplied by a tankless water heater. The tankless water heater is acting peculiar.

I'm getting hot water out of the shower heads (directional, rain, and hand showers) but I'm only able to get lukewarm, not hot, water out of the tub filler. I've even experimented with the hot water temp on the unit to see if it just wasn't high enough, I cranked it up slowly 5 degrees at a time up to 140 and I still was only getting lukewarm water out of the tub filler.

My contractor seems to think that the flow of water into the tub is too fast which is not providing enough time for the water to be heater properly. A Reem unit was originally installed which has now been swapped out for a Steibel. Both exhibiting the same problem. My contractor is now pushing for us to just do a tanked water heater instead. Should I have any confidence that this will fix the issue? Does it seem like the unit(s) are just improperly configured/connected?

Thanks for any and all responses!