The undyed shulker box uses the same tint colour as the purple shulker box:

I did some digging and I feel like the problem is somewhere around here:

• https://github.com/maruohon/malilib/blob/b3f0c629caa192bd78fb81ad978e71e1ace4be68/src/main/java/fi/dy/masa/malilib/render/RenderUtils.java#L1168

Maybe something changed in 1.14?

Correction: it looks intentional. Instead of returning purple, I think the GUI should remain untinted, just as the shulker box is undyed. Perhaps instead of returning EnumDyeColor.PURPLE in this case, short-circuit with color(1f, 1f, 1f, 1f) just like other parts of the code seem to do.

I made this change in maruohon/malilib@4a85ae6 which will eventually get merged to 1.13.2 and 1.14.4.