I have this pack that adds a swirly nether portal animation to ender chests. 90% of the time, the animation "frames " fails to render and all of then gets squished into one, yet there are very rare ocurences where it works normally if I reload the packs. I included the textures, the ender_chest.jem file, and examples of the issue and it's expected appearance.
the pack is: https://www.curseforge.com/minecraft/texture-packs/chests-reimagined