Affects Version/s: Minecraft 1.13-pre4, Minecraft 1.13, 1.14.4, 19w36a, 1.15.2
Fix Version/s: None
I say "towards" a log because that's due to the order the game places the blocks. It places them from the lower coordinates of an axis towards the highest (except for Y maybe, it's a little weird, I'll talk about that later). Because of that, if the last block of leaves that the game creates is the first and only one to touch a wooden log, the first leaves were created without being "connected" to a log, so their distance is set to 7. And when the last block of leaves is created, they're now all "connected" to a log, but since it's the last block created, none of them (not even the last one) is updated.
If you're standing on the log, this issue happens when you create leaves :
• Towards negative X
• Towards negative Z
• Towards negative X + negative Z
• Towards negative Y. Which means here that it will be for an upside-down "tree" (and that for this axis, blocks are created from higher blocks to lower ones). However, I've at first been able to do so by hovering some blocks over the log and filling leaves between me and the log, and there was the issue, but for some reason I'm now completely unable to reproduce that again... I have no idea why, I can only show you where I managed to do that :
But at that moment, what happened was something that made me think of something kinda different for this bug : creating leaves under me to connect with the log would produce the bug, and creating leaves above me while sitting on the log would not. And I don't know why, now none do.
Note : Adding the tag "distance" set to a value between 1 and 6 allows to get round the bug. But it also means that leaves with these values seem to check better for an update.