If 2x2 trees can be grown in the Nether and The End, why not under y = 0. I am guessing this is a bug.
I even made 8x8 hole thinking it it was about the light mechanic.
I can grow 1 sapling trees just fine under y = 0.
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is duplicated by 4
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Video demonstrating reproduction steps can be seen here: https://youtu.be/Fg9NyZh0aHw
Also affects 2x2 jungle trees as well, dark oak however is not affected.