Certain items, when remodeled, render differently than other items. For example, stained glass items will display both sides of faces even when remodeled, however other blocks such as quartz blocks will only render one side of faces. This is inconsistent, and seems to be due to the special cases defined for transparent block-item models. This means different items may not display the same despite having the same model.

The attached image displays two items, white stained glass on the left and quartz block on the right. Both have the same model, however, they render the model differently. On the custom stained glass model we see faces from both sides, however on the quartz block model we cannot.

Perhaps a way to resolve this would be to allow models to define in json to be rendered as a transparent model/double sided, then models could be rendered in the correct way regardless of item.

The resource pack for this has been attached, simply drop white stained glass and quartz block on the ground to see the differences.

Note: Additionally, partial transparency does not behave correctly either on default opaque item models but work correctly on originally partially transparent items like stained glass.