Is it a mod compatibility issue?

Yes

Minecraft version

1.20.1

Describe the issue

Having a regular portal loaded will cause no problem, but when re-lit to a immersive portal all chests become invisible. Disabling Iris fixes the problem immediately.
Not sure if this counts as being "vague", but I don't know what else to add.

Which shader are you using? Does this issue occur with all shaderpacks?

I am using Complimentary Reimagined r2.2, but after switching to BSL Shader v8.2.04 the chests are visable again.
I have noticed that the chest-invisibility actually applies to all entities. Also, it only renders entities from one dimension at a time. If the overworld's entities are visible, the nether's aren't and vice versa.

Same goes for me, but i'm not sure it's due to ImmersivePortals tbh, it started quite randomly, idk why...

I would argue otherwise, since it only happens when I enable ImmersivePortals, and has not occured any other time.

I'll trust you then, i'm a shit in these things, atleast i know where it comes from ! Luckily switching between 15 and 16 rendered chuncks fixes it for the zone you're in, so that helps for now i guess xd

Can confirm.

Item entites aren't rendering:
Signs
Chests
Item Frames
Armor Stands
etc

I love this mod so much I *almost don't care lmao.