I have never seen this.
I planted a series of 10 trees in one go. I am chopping them now. 8 of them have fully grown, and provided cuttings, yet two seem to not yet have fully grown, as I can't strike cuttings, or bark from them yet.
It can't have any connection to different types of tree. I already harvested two birches, and the two that haven't grown yet are also birches.
It would make some sense if the two trees were the last 2 planted in the series, but that's not the case either. They are about in the middle of the row I planted.
Is there a slight random factor now to tree growing time, or is this buggy behaviour?