This bug is sort of Bukkit/Spigot related but the issue lies in Minecraft.

If you spawn a falling STATIONARY_LAVA block through the Bukkit API, the block will not be visible as it would have been in Minecraft 1.7.10. That said, the entity is still there (able to track its location, add surrounding particle effects, and affect entities that come too close) and a shadow forms when the falling block approaches the ground. Yet the block itself is completely invisible.

I came to wonder if this was a problem with textures, until I found this bug report: http://www.reddit.com/r/admincraft/comments/2ejbm1/psa_minecraft_18_renders_blocks_with_invalid/

This is pretty much what I've observed with the falling block entities, except that I've tried every possible byte data for lava (from zero to 15).

Not too sure what causes this but it most certainly is a bug, and one that I myself, and probably a lot of other developers, would like to be fixed.