I was trying to set up a piece of UI where buttons are laid out horizontally inside a ScrollingFrame. The ScrollingFrame will only scroll in the X direction. Unfortunately there is no way to make mouse wheel inputs cause horizontal scrolling, even though such behaviour is fairly standard and intuitive for similar UI in other games. An option to allow mouse wheels to perform horizontal scrolling would help make for UI that is quicker and easier to use.
A fix for this is shipping next week.
The fix for this is now enabled.
I encountered this same exact issue today. Using my scroll wheel would work maybe 5% of the time.