Salem

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?

sounds buggy to me, all my trees have grown at the same rate.

Trees grow to full size at the same rate but there is some randomness to when they produce things like bark, nuts cuttings, ect.

Well, it seems those two trees took about an hour longer than their team mates to mature. They are done now. Must have been odd bugginess then.