I know there is one in the contributor repository called bloxel or something. Not sure about any others, though.
I think the reason there aren’t many is because it’s actually really hard to make one. I think people see blocks and presume the graphics quality relates somehow to its simplicity. In a way they’re right; the amazing thing is how the multitude of problems were overcome in a simple and elegant way, but those problems are vast and complex in their design. Greedy meshes, cellular automata, picking, ambient occlusion, materials, data packing, water simulation… the list goes on and on. In all honesty it’s one of the most complicated of games I could think of to attempt. I’ve seen many tech demos but none that have them all except mythruna.
It might be quicker to YouTube some searches for jmonkey voxel demos and see if they are signed up here to ask them directly.
If all else fails you could take a look at Paul’s muthruna and use a java decompile to see the code as an educational reference to see his design structure; it often gives insight.
Other than that there are open source implementations for Minecraft servers and clients that again will help you understand how all of these problems were solved. The Minecraft wiki also helps a lot. It explains how they pack data and again provides insight on the inner workings.
I realise this isn’t the answer you wanted, but these were the processes I went through whilst making a terraria clone for fun.
jmonkey contrib repo
Spout - minecraft client
Paul Speed’s voxel game written using jmonkey: http://mythruna.com/
Minecraft wiki chunk format (an example of freely available information): https://minecraft.gamepedia.com/Chunk_format
SpigotMC - a plethora of voxel-oriented java code: