When seeing inside of blocks (like you normally would in spectator mode), some of the blocks are rendered like... not quite solid? They still block your view and suffocate you when you're not in spectator.

From what I've tested, it seems to be mostly metal blocks
More specifically, it seems to be the case with:

• Thallasium and terminite blocks, including tiles, stairs, slabs
• Smaragdant crystal blocks
  

Aeternium blocks are fine btw.