I noticed this whilst doing some resizable graphic tests.
This doesn't happen with all nine-sliced objects, and I think it has something to do with the position of the slice. In the following example, the "splat/antisplat glow" objects have a RIGHT_20 and LEFT_6 slice for resizing purposes (ny less than that on each side results in an inconsistent graphic due to shading):
(https://i.imgur.com/vh2Epnf.png)
This only affects the graphic in the editor; it appears fine in-game:
(https://i.imgur.com/1AFZOCt.png)
I have attached the png and nxmo for the antisplat glow in case the fault is with either of these (the splat glow is identical but recoloured to red).
Almost certianly an editor bug, it wouldn't be the first time it's had issues with resizing.