When making an automatic melon and pumpkin farm using observers, when one of the plants grows and places the block the observer does not detect a change. On Java edition this isn't an issue.
I believe this issue is on all Minecraft Bedrock versions as I was able to replicate the issue on iOS and Xbox One.
Issue on iOS:
Linked issues
Comments 2
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-36783.
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See MCPE-36783.