First, to get any kind of accuracy with an infra-red thermometer you can't be looking at bare metal. Either paint the patch of plumbing with a flat black paint or wrap a short section with hockey tape and measure the temp of the tape, not bare pipe. (PEX is reasonably IR-emissive and need not be wrapped, copper/brass/bronze/galvanized is not.) You may still have a comparable delta-T, after giving it an emissive surface to read (or not) but at least you have a shot of taking good relative-temp measurements.
There is a significant pressure drop across any tankless at high flow rates, and that only gets worse with time due to lime-up issues in hard water areas. As the pressure differences between the cold side and hot side get bigger, so to the interactions with anti-scald valves, but lime up can also affect temperature stability (particularly at low flow.) If you haven't already, descale the tankless (not just flushing the lines- it needs a mild-acid such as white vinegar to break down the scale, circulated for 15-30 minutes through the heat exchanger using a small pump. Don't forget to turn the power off to the unit while descaling lest you experience the joys of steam-driven jets of boiling hot vinegar spray.) In hard water areas descaling is often at LEAST an annual maintenance chore with tankless HW heaters, and often needs to be more frequent than that, depending hot water use and absolute water hardness levels. In these cases it's worth installing isolating ball valves on both sides of the tankless, along with ports for circulating a descaling solution through the tankless.
The high static water pressure is not likely to be related to the problem.