First Rotatesprite Interp comes from center screen with 0 scale. (RS_FORCELERP)
If RS_FORCELERP is on, the first time a sprite with a particular guniqhudid shows up, it interps from the center of the screen, with 0 scale.
Case in point here. I'm using RS_FORCELERP on the Flamethrower's upper segment. First time I fire, you can see how messed up it is. Second time, is fine.
This behaviour can also be observed in the Vanilla game with the shotgun. First time the shotgun's muzzle flash shows up, it shows floating at the screen's center for a frame or two.
My guess, is that the first frame a new guniqhudid is displayed, it needs to have the old position set to match.