Placing a dirt block over the oak sapling to prevent big ones from spawning, will, most of the time, result in them spawning on top of the block, leaving the sapling untouched
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Can recreate, lol

good find!
Thanks a lot for your ticket! Someone already reported this earlier in MC-164143, but your ticket helped me to find the cause of this.
But I'll resolve this as a duplicate of MC-164143, since we don't need two tickets about the same ticket. Feel free to vote for that ticket and leave any potential information that you might find there.