As you can see, item_model is set to uhc:shining_berries. It should be set to minecraft:glow_berries, which is the Item returned by getPolymerItem. I didn't see any changes in the documentation, so I think this is a bug

I looked at the code, and it looks like the easiest fix would be in java/eu/pb4/polymer/core/api/PolymerItemUtils.java:503:

return new ItemWithMetadata(out, lastVirtual.getPolymerItemModel(stack, context));

if the second parameter was changed to out.getPolymerItemModel(stack, context) or the getPolymerItemModel was changed to return the model of the vanilla item instead of the modded one, that would fix the issue.

on second thought, the fix I suggested will not work at all. I fixed it in my own mod by overriding the getPolymerItemModel method to display the correct model of the polymerItem.