Tested using the following as basis:

Minecraft 1.19.2
Fabric Mod Loader 0.14.19
Mod Menu 4.2.0-beta.2
Sodium 0.4.4 + Sodium Extra 0.4.16
Iris 1.6.1
ImmersiveMC 1.3.1
MC VR API 3.0.2
No Tree Punching 6.0.0
This Rocks! 1.6.1

When any semi-transparent block is placed in front of a furnace that has items in it, the ImmersiveMC previews of those items will not show at all. Such items may include pressure plates, grass, rocks (No Tree Punching), pinecones, sticks, sea shells (This Rocks!) etc.

See examples below.

Rocks (easy to miss):

Grass (furnace placed outside earlygame):

Pressure plates:

For my future self handling this:

This is NOT fully fixed by the newer air check. It does handle things like tall grass, since that's a replaceable block, but not blocks such as pressure plates. I think it'd be pretty reasonable to see if the height of the block shape is less than some relatively low value, and if so, don't block the render. This way, things like slabs do block, but things like pressure plates don't.

Thanks! I had run into this before, but didn't open an issue for it for whatever reason.

I definitely want to fix this. Will probably look at the shape of the block in front of the furnace, as imo, blocks like tall grass should block the furnace at is massively obscures it.