All shower valves are required to have one sort of anti-scald technology in them. The most common one is pressure balance. To work properly, the pressure on both the hot and cold must be within specs and ideally, equal. It is not uncommon for a tankless system to have internal flow restrictions, meaning that the outlet pressure could be lower. This can be enough to cause the spool valve to close, shutting down any flow. Depending on the tankless system, you might notice it getting worse and worse as the inlet water temp decreases as winter approaches - some tankless systems restrict the flow so that they can transfer more heat which maintains the set point on the outlet temperature.
You might find that reducing the set temp on the tankless would mean that it won't need to reduce the flow to produce the desired temp, making it more equal with the cold supply, and thus keeping the spool valve centered and not shutting things down. This can be annoying, depending on what you need for outlet temp (typically, you'd want more for washing dishes, and maybe for the washing machine, if you use hot for it).
Try dropping the thermostat on the tankless to something like 105-110 degrees and see if the problem still exists. that means that you'd be using almost all hot in the shower and you MIGHT need to readjust the max temp setting in the valve to allow it to get hot enough for you, but as a quick test, it would tell you if it is the balance in pressure between the hot and cold.