This is only a graphical bug and does not affect gameplay in any manner.

When you place a melon/pumpkin by their respective immature plant the plant will connect to the block as if it is full grown. This does not affect the plants growth whichI tested outside the video after adjusting the random tick value to 1000 (I'm impatient).

Steps to reproduce:
Simply place a pumpkin seed by a pumpkin or vice versa (same with melons).