There is a known issue with x20 hardware which limits how Still Layers can be used in certain High Rez setups.

When configuring a frame with a VI Vertical Pixel count above 1850 pixels in Height, the system becomes limited with how it can utilize Still Stored images.

Only layers 3,6,11, and 14 can be used as still layers. Using other layers will induce strange behavior, including cropping, random noise, and other various errors when adjusting Keyframe parameters.

This is a hardware limitation with all x20s and all versions of Vista Advanced.

TL;DR-Exact wording via Vista Systems: "If the vertical height is higher than 1850 only layers 3,6,11,14 can load stills. This is a hardware limitation for any version software."

I have also heard people suggest this limit was at 1856 pixels. YMMV.