Tankless hot water shutting off when more cold water is needed

I have 3 tankless Bosch heaters in my house.. one for each apt.
2 of them are having the issue where the hot water goes cold
after needing to turn the cold water up to make the water less hot.

The hot shuts all the way off.. Do I just need to turn the temp down on
the heater so I don't need to add as much cold to make the water the right temp?

Is there any chance the water is running backwards? where the cold would
push the hot back?

thanks Noob in New Orleans

Several of gas-fired Bosch heaters have a minimum-modulation of ~30,000BTU in, and if you cut back the HW flow too much it drives the output temp up to where the overtemp safety trips. The warmer the incoming water is, the more likely this is to happen. The work-around is to run higher flows of the hot side even as you mix in more cold water to keep the temp at the tap reasonable.

And yes, at the local mixer increasing the flow of cold water introduces a back pressure on the hot feed, slowing down the flow.

Assuming there is a flow restriction at the point of use, Adding more cold to the mix would reduce the flow of hot and could drop below the minimum flow rate required to fire the heater. Instead of asking the question, why not just turn down the temp and get your answer that way?