For a start, it's a modded scenario, although the issue itself is pure vanilla (the model loader was NOT modified in any shape or form!). Please, don't put me down for it, required to explain how I found it.  I'm also posting this because you want to be as supportive of modding as you legally can be.

I have registered several shields, and noticed that the model loader will ignore property overrides, (in this case "blocking") that are coming from the parent model. It's confusing and may cause issues later down the line.
I'm including the models, shield.json would be the parent that was meant to be inherited by all shields, overriding 1 texture.