Melons and pumpkins do not connect to fully grown stems if you add the pumpkin or melon next to the stem. This causes issues with testing automatic melon/pumpkin farms as if you are using a observer on the stem, it won't update. On the other hand, pumpkins and melons that are produced BY the stem do connect. Direction that the melons or pumpkins face does not affect this behavior. I have attached an image showing this phenomenon, with the farthest back melon being one that grew on its own, and the other 4 being ones that I have placed.