There are some Rendering issues with the inventory preview of shulker boxes. the corners are extended bacl, an the top border is even worse.

I assume you are using a resource pack that changes the inventory background texture? Can you send a screenhot?

It's on my TODO list to redo the background rendering, but I'm not sure if it can ever work correctly for any arbitrary resource pack, maybe I would have to just add the vanilla texture in the mod since it at least won't break then.