9-slice UI is bugged up in WebGL [2018.4.1f1]

So, I have some UI graphics set up for 9-slice use. Originally I had them packed into a sprite atlas along with other elements, but that caused glitchy visuals in builds, so I took them OUT of atlases. They look perfect in Unity editor, in Android and in Windows builds, but in WebGL they do this weird thing:

Using Unity 2018.4.1f1




Any ideas??? Why would a 9-slice UI image map to a completely different source texture in WebGL builds and not in other platforms?