The part of the block model that extends beyond 1x1 becomes black when embedded in other blocks.
Little impact on the vanilla but very unfriendly to resource pack authors.
With XeKr Light Color and 3D model pack:
https://www.curseforge.com/minecraft/texture-packs/xekr-light-color/files
[media]
I could not reproduce this with the resource packs provided. Do you have some different packs that this could be tested on?