This issue is really simple to reproduce, so I won't need to go into any depth.

Happens with and without shaders. I am not using Optifine, but the modpack I mention below has Oculus.
It doesn't matter whether the block above the mat is vanilla or not.

Edit: Same or similar issue: #3613

I'm on the java client, 1.18.2 and using the Better Minecraft Plus Edition for Forge 40.1.25.
My graphics drivers are up to date and I have added a few mods to the pack:

• FallingTree
• Easy Paxel
• Coreil Backpack
• Coreil Deconstruction
• Coreil Recycler
• Falling Leaves
• Grappling Hook Mod
• Immersive Engineering
• Immersive Posts
• Simple Planes

Can't reproduce, can you get this without Oculus?

No. Without oculus, everything is fine. As soon as I install oculus + rubidium, they go transparent.
Carpets for example are fine though, with or without oculus.