Mods like Gregtech have some Multiblocks that are only valid if there are hollow (only air inside).
Lightblocks from "Feral Flare Latern" inside such multiblocks leads to invalid structures.

Please see: GregTechCE/GregTech#454

I did some testing and it seems its a bit more complicated than that.

I register my block with Material.AIR. Block#isAir() simply checks if the material of the block is Material.AIR - therefor, I already return true for the isAir() check.

As is stands right now, I dont think I can fix this easily, if I can fix this at all.
It seems gregtech doesnt check for Block#isAir() but for the blockstate - which is obviously different from air.

Sadly, i cannot fix this issue on my end. I'm already doing everything I should. Gregtech needs to handle pseudo-air blocks properly. Sorry :(

If you have anything else, feel free to create a new issue or respond to this one. I'll close this for now.