The bug
Observers do not update upon tree growth unless they were already observing the sapling.
Also, block states are not updated properly, for example fences do not start connecting.
Code Analysis
A code analysis by @unknown can be found in this comment in MC-136442.
Linked issues
is duplicated by 5
relates to 4
Attachments
Comments 29
I can't confirm this. It seems like 2x2 trees need the surrounding airspace in order to grow, so unfortunately I can't think of a way to test this. If you have direct evidence of this still occurring please do post it here
This does not affect 1.12.2