The bug
The hitboxes on melon and pumpkin stems are too small when compared to 1.12.2, especially when they are in a non-matured state. In the first state, they are much smaller compared to their textures. The same goes for most of the other states. The hitbox on the matured stems seems to make sense but it could also use a resizing as well. Just increasing the sizes of the hitboxes of the melon and pumpkin stems would fix this bug.
Confirmed for 1.13.2-pre2.