I know this has been fixed on the Java Edition, but this doesn't seem to be the case on the Bedrock version.
Linked issues
Comments 3
I think that this works on Java because of the F3 screen showing that the stem has already grown a pumpkin/melon. Since there is no F3 screen in Bedrock, I suspect that this isn't a feature.
Thank you for your report!
We're actually already tracking this issue at MCPE-36783, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature, to see if the issue has already been mentioned.
Suggested search terms for JIRA desktop view: {color:#009900}mcpe unresolved pumpkin* stem* observer*{color}
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki
DesertTortoise23
No Expert
I don't remember them ever detecting stem growths. I experiment with observers all the time.
Don't take my word for it though