After playing on a survival server running a modified version of Prominence 2, sometimes a player's wings will roll them left and right with the mouse movement, but other times will just function as a turn left and right without the roll. At first I thought there was a control or keybind at work here, but I can't seem to find any icarus related controls.

It's likely an interaction with 'Do a Barrel Roll', not relevant to Icarus. I'll try to figure out how to tweak that.