What was the cause of this bug?
Optifine issue
Optifine has a limitation which doesn't allow custom armors to work correctly if you have any custom Optifine shaders installed.
You have to disable the Optifine shaders temporarily or temporarily live with the issue.
I already contacted Optifine developer about this: https://github.com/sp614x/optifine/issues/6391
Iris Shaders issue
Iris has a limitation which doesn't allow custom armors to work correctly if you have any custom Iris custom shader installed.
I already contacted Iris developers about this: https://github.com/IrisShaders/Iris/issues/1042