As title. The placed chest becomes invisible but still useable.
1.12 v1.6.1

Could you provide a gif of what's happening?

The check is made by the block itself, meaning if you pick up flowers they can only be placed on dirt or grass. If modded chests don't override this check with their own, there's nothing I can do.

No idea how i make a GIF.
Tried around a bit more. Seems to be a problem with chests from mods.
vanilla chest is working (not placeable beneath a double chest)

Chests from "Quark" (jungle, acacia, ...):
Place a double chest and a picked up quark chest (same type) like that:
XX
-Y
the Y chest becomes invisible and has inventory like a double chest. (shared inventory with the chest at the edge)

edit:
I guess the problem comes from your mod checking for only "minecraft:chest" on placing. If you could add oreDict "chestWood" to that check (and then check if the same chest is placed there as a double chest already) it should work with chests from mods too.

What check are we talking about here?

Block#canPlaceBlockAt. Are you doing the placement check on the itemblock?