When an observer block is placed in front of a melon or pumpkin stem, the observer will not notice the state change of the stem as the melon or pumpkin is generated next to an adjacent block.

To reproduce, here are the steps:

1. Place melon/pumpkin seed on farmland
2. Place observer in front of stem
3. Notice that the observer generates a redstone signal as the stem grows
4. Notice that the observer will not generate a redstone signal as the pumpkin/melon is generated and the stem gets attached to the fruit beside it.