While TNT mining in the nether I noticed that the destroyed blocks turned into invisible blocks.
OR it produces blocks that are blown-up, can be flown and walked through, but render like a normal block from the outside. They act like xray when walking into them.
Placing a block or reentering the nether fixes this.

To reproduce go blow up some stuff in the nether.
Might only work, if there is air below the blown-up netherrack. idk.

1. pic: from above
2. pic: from below
3. pic: standing in front of a wall
4. pic: after walking into that wall