Melons in the current snapshot very, very rarely generate within the Jungle biomes. On average, melon clusters appear to generate about once every 100 blocks or more. Expected generation is for melons to be in all the Jungle biomes in a noticeable quantity.

How to replicate: go into a world, teleport to the Jungle biome, and try and find a patch of melons (its surprisingly hard to find!). In checking out the other jungle biomes, especially in the sparse jungle, make note of how few melons can be found. (it took 3 separate jungles to find a melon patch in a jungle biome, and a sparse jungle next to this jungle ended up having no melons).