First off, the previous issue was marked as resolved because I erroneously put the wrong name in the testing plugin; this is now correct, and I've double-checked that this is an issue that is still occurring. I apologize for my mistakes in the previous issue.

When using ItemMeta#setItemModel, only the item in your inventory is retextured. Retextured armor does not retain the new texture when you put it on your head (although it still appears to be retextured in your inventory).

Here is an example plugin:

https://github.com/Dartanboy/TextureTest

Here is the texture pack I used for testing:

https://drive.google.com/file/d/1A7s0iYSFIOjZW6tI8jEabDcsfzEQ81M5/view?usp=sharing

It should be noted that using a /give command does still retexture the armor. Here is a video of me using /give (and the armor being retextured) and then using the plugin to retrieve the item, which does not retexture the armor:

https://youtu.be/lTvL0sSKALc

Here is the /give command: 

/minecraft:give @s diamond_helmet[item_model="oni:oni_1_helmet",minecraft:equippable={slot:"head"}]