Messing around with the .JSON, i figured out what triggers this. Don't know why it is a problem with Forge.
The texture pack uses a custom item model, so the panes display in the inventory at an angle-- looking like a flat object that they are, instead of an inventory space just filled with a color.
A snippet from the file:
"gui": {
"rotation": [ 30, 225, 0 ],
"translation": [ 0, 0, 0],
"scale":[ 0.75, 0.75, 0.75 ]
},
Messing around with it, i found If i changed "255" to a lower number, for instance "45" everything displays as desired.