The arms on any DTI clamp will cause the reading to change if you knock them as the indicator is very sensitive.
But perhaps that is not what you meant and the base is not flat?

Maybe they are not genuine Mitutoyo ?

I bought a couple of the ‘red’ cheap stands off eBay years ago and had to dismantle them, lightly machine the mounting faces before using as they were not flat. The V looked like it had been hand linished to no particular shape.