When you make a custom model for an item that doesn't use transparent textures (e.g. White Glazed Terracotta), the textures will render opaque.

Edit: This only affects items of blocks (e.g. Glazed Terracotta or Carpets). Remodeled items (e.g. Iron Ingot) work fine.

In 1.14.4, the textures would render transparent as they should.