The BR frames for 6 thrusters vs 8 thrusters are identical except for an extra two vertical thrusters in the 8 case that control pitch, so that shouldn’t make a difference to any part of the end result other than if you were using pitch control.
This could be consistent with the ‘masked values were being accepted’ issue that @williangalvani described. Another way of testing this that might be a bit safer (but makes control a bit more annoying) is always setting all of the RC_OVERRIDE values instead of using the 65535 mask value for the ones you’re not changing. Try setting all the values to 1500, except for the one you want to be something else. Hopefully that will force the thrusters to be still other than the ones that move the ROV in your desired direction.