Prior to the release of 18w43a, the attached melon/pumpkin stem model placed two small parts of its stem texture towards the bend in the stem to give it the appearance of having branches forming at its root. However, in 18w43a, the model was changed to move both of these 'branches' to one side of the model, now facing to the right of the attached block. This gives the stem an asymmetric appearance and doesn't really make sense when contrasted with the stem's prior appearance.

I've attached an image of the stem before and after the change. (Disregard how one stem looks slightly darker than the other - this has to do with other changes made to the model.)