I've run into what I think is a bug with virtual scrolling and adding backdrops. I don't think it's specific to iOS, but I'm unsure so I thought I'd post it here anyway.
What I've got is a frame with virtual height = -1, and scrolling coefficient for the layer = -1.0
Basically there are two problems, firstly the backdrops are sometimes not placed in the correct place (see 1. on the attached image). They are offset by a strange amount, even when the active is in the correct place. That is to say, looking at the code, the active is only ever "moves" 40 pixels up or down, yet the image is not pasted in this area.
Secondly, there is some "smearing" occuring which can be seen by the motif quick backdrop on the right. (see 2. on the attached image)
The behaviour seems to depend a lot on where you place the active initially as well. If you place the active in a place where its Y position is a multiple of its height (e.g. at (0,0)), then the smearing and glitching doesn't seem to occur.