I using a APM with an X8 (wing with elevons)
R/C controls function fine in both manual and stablise modes.
The problem is in stablise mode, when the aircraft is banked to the left, the left elevon should go down, and the right elevon should got up, to roll the aircraft right back to straight and level. And the opposite when the aircraft is banked to the right.
What actually happens is that the elevon that is supposed to go up, does go up.
But the elevon that is supposed to go down, only goes down to neutral position, and doesn't go down enough.
R/C controls have been calibrated and manual R/C control in both manual and stablise results in full deflections both up and down.
Pitch stablisiation works fine, the elevons move up and down full deflection.
I use the same modell, now with the apm2 too. Not maiden'd this combo till now, but the behaviour you explain is imho the differential ailerons. I have the same behaviour here. As far as i know this diff. ailerons are neccessary to get correct barrel rolls only by using ailerons. If you dont have this diff. ailerons acting you have to compensate with rudder or elevator to get correct rolls etc. I agree that fpv with a X8 is not really the scenario for barrel rolls or something like that, but its in there now. ;)
In the APM planner, in the Radio configuration section, use the "reverse" checkbox/s next to each radio channel to make them work the correct direction in stabilise mode.
i suggest just reversing the "aileron" channel. ( or what would normally be aileron if you had them ).