Never read of that problem before with SX, I assume you have checked all the obvious like FC center of the vehicle facing up and forward, no major electrical interference present. the level is taken from the FC not the OSD, the vehicle was in flight when you took the picture what about putting it on a level surface
Correct, the weird thing about it is that when I first power up, the line will be level, but after 20-30 seconds you can watch the line slowly going off the level position. That picture was taken while hovering, in a no wind condition.