Unrendered json textures

[Iguanoddon]

I found that some json textures and models do not load properly with the texture pack linked bellow. The corresponding blocks are then replaced with invisible ones in the scene. Also note colors in the Map view are wierdly rendered (grass for instance).
The pack : http://www.amberstonedream.com/en/pack1.php
I tried HQ versions : 1.14, 1.15 and 1.16
Examples of unrendered blocks (you'll need worldedit to place some of them) :
99:15 = mushroom_stem (probably based on block orientation)
99:1 = brown_mushroom_block
birch_fence_gate
probably a lot more...