Observers don't update when a sapling grows next to leaves. If a player breaks or places a log that is connected to leaves, the observer updates, but trees that grow will not update the leaf block in the observers eyes.
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galaxy_2alex
Arisa Bot
Thank you for your report!
We're tracking this issue in MC-180771, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as Fixed. The fix will arrive in the next version or is already included in the latest development version of the game, you can check the Fix Version/s field in that ticket to learn more.
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This is fixed in 1.16.