The bug
If you break a fully grown pumpkin stem or melon stem that is attached to an adjacent pumpkin/melon, no seeds will drop.
This also applies to entities trampling the farmland, breaking the attached stem with a piston, water washing the attached stem away, and /setblock
destroy.
Also relating to melon and pumpkin stems: if you place a melon or pumpkin next to their counterpart stems, the stems do not connect with them like they did in 1.12.2. Don't know if this is intended or if I should make another issue for it.