Why do I have this plane and why is my cup, in Object Mode (Principled BSDF, Transmiission=1, Roughness=0) have what looks like an HDRI country scene in LookDev mode?
You do have another plane in your scene, it's just not showing in the viewport.
In your outliner check these options and look at all the collections you have, there is a plane there not visible but renderable. You can disable by object or entire collections.
Nice donut btw!
More about it at: Blender Docs