XB8, yes flat terrain, offroad but flat.
I think external factors might be a consideration too, have you tested in a different location completely?

I'm not sure if the radio makes a difference either, this was with an MT4S which does run a higher TX voltage than the MT44/M17 (2S vs. 1S).
I wouldn't expect it matters but haven't tested with my MT44.

If you want to test if the orientation is causing it, you could just mount the RX outside/above your RX box temporarily and see.
Worst case though I'm sure you'd find a trade for a 471/472 quite easily as long as it's a genuine 482.