Yep, most of my blocks are custom models, so I set isOpaqueCube and isNormalCube to false almost always. It looks like the Block#getRenderType already returns EnumBlockRenderType.MODEL, so setting it like that should not be necessary.
It might be related to the material you set for your block. You probably want a material that is "translucent". So I would suggest creating a new MaterialTranslucent() with whatever map color you want and also probably setNoPushMobility().