I don’t believe that thermometer is designed to measure small pieces of meat. From the equivalent Amazon page:
“For best results, slide the entire shaded area of the probe into cooked meat, avoiding large bones.”
If you want to measure something of that size, I would look for a thermometer that takes it’s reading at the very tip, rather than what appears to be the average over a large portion.
Highly recommend something by Thermoworks (Thermapen, MK4 or ThermoPop). They’re accurate, very fast and read right at the very tip. (No affiliation, just love the product).
Aside from that, I typically wouldn’t use temperature to check whether a stir fried (small dice) piece of meat is cooked. I’d probably just cut one piece open and look for it to be cooked through. If it’s small enough, by the time the outside is at the right colour, the inside should be cooked through. If you’re not confident, and really want to check the temperature, you’ll need a different thermometer.