The bug

If you use a debug stick to set TNT's "unstable" blockstate to true and then ignite it, any blockstate textures defined in tnt.json are lost. There is an example video attached below.

How to reproduce

1. Install the resource pack in the attachments.
2. Give yourself a debug stick with /give @p minecraft:debug_stick
3. Place down TNT and right click it with the Debug stick so that unstable gets set to true. The texture should've changed to be text saying "This texture is lost when ignited".
4. Ignite the TNT. You'll notice the texture is replaced by the default TNT texture once it becomes an entity.