Test: Create a world in creative, log a glass pane in a wood slab (can be any kind of wood)
Save world and quit to menu.
Reload world, windowlogged block is completely invisible, but you can't place another block there, so it's clearly still there but simply not being rendered.
You can fix this by using a setblock command to air on that block, if you choose 'destroy' it will pop the slab off, showing that it's sitll there (you don't get the pane back for obvious reasons.)
Forge: 38.0.15 Windowlogging: mc1.18_v0.0.5
Test: Create a world in creative, log a glass pane in a wood slab (can be any kind of wood)
Save world and quit to menu.
Reload world, windowlogged block is completely invisible, but you can't place another block there, so it's clearly still there but simply not being rendered.
You can fix this by using a setblock command to air on that block, if you choose 'destroy' it will pop the slab off, showing that it's sitll there (you don't get the pane back for obvious reasons.)