I've reated my whole world in 1.15.2, so this isn't an update problem.

When exploring world I founded, very rarely, trees cutted, or generated, in half.

the only thing I did, which makes me think it may have caused the bug, is that I rarely use the world optimization function to improve the loading speed.
Maybe this bug is related to world optimization feature?

This bug happens only with trees.
The only fix is to destroy completely the trees and make them regrow by planting samplings.