This issue has been happening for a while, it is just hard to catch while playing the game, so the report is delayed.

I have the pumpkin/melon farm setup as seen in the screenshot. And I have a nether portal very close by. Occasionally, when I come back from the nether after Spending quite a bit of time there, I find the farm in a state that everything has grown, including the pumpkin in front of the observer, indicating the observer has missed the event of pumpkin growing. I had previously planted a melon in that spot, which also had the same issue.

This issue seems to affect only this farm, it could be based on location of it, or the pumpkins/melons. The other observer I have is in a sugarcane farm, which seems to function correctly.