This bug has persisted since the magma cube was introduced and has yet to be fixed. Here you can see the texture mapping showing the pieces of the model, but instead of having 8 top and bottom cubes for all the inside layers and the bottom like how it's shown with the patty and tomato in the bottom right of the texture .png the texture mapping is set to overlap the top and bottom image of the top and bottom of the whole mob, resulting in an overlapping texture on the inside for a majority of layers (tomato and patty excluded because they happen to have their own top/bottom section in the bottom right)

I'm unsure if the mapping defaulted to this setup to fill each part of the model because it didn't have all the sides it needed available, or if the overlay is manually made to fit the image and paired with the model through coding. The texture file itself doesn't need to increase in size, I took the original, moved things and duplicated the current tomato and patty cubes to make the amount needed to fill the whole model. The blank space in mine being where the inner cube texture would go. The file itself is a perfect size, nothing has to be rescaled or resized. 

In vanilla the magma cube is majority of one color and texture, a texture that when overlapped can look fine, but if it were changed ever, it would be very noticable. This issue especially impacts the community members who make resource packs and texture packs because they can't utilize the unique layers or this mob without it ending up looking, like a mess. 

I'm all for limiting things to spark creativity, but this should not be something that contributes to the limitations.

Now with 1.20 coming up, and the fact that a big part of the theme seems to be self expression, I think this is the perfect time to fix this. Not only is it a simple fix of tweaking the texture .png to actually be fully complete (which you could just use mine, its not like its an original idea or anything) but to just fix the texture overlay. Heck, If I knew how I would do it myself.