but is there a way to fix the rendering issue (grass block under the jukebox not rendering)?
Yes. Make it a full block like it was intended to be.
Minecraft uses a fairly common technique called "culling" to remove faces that it doesn't believe should be visible. This saves the games from having to render millions of block faces that the player can't even see, which would of course make the game completely unplayable.
Unfortunately its concept of what should and shouldn't be visible is hard-coded - you can't change it with a resource pack. As such you need to construct your models with this in mind when you're making a pack, or else replace a block that's already transparent such as glass.