I have been trying to figure out what is causing this issue in my modpack but I have gone through every mod I can think even might be causing it, tried several search queries on it, all with no success.

Does anyone know what mod is causing this incompatibility?

(ps : none of these leaves are decaying and I haven't broken or placed a single block in this world yet)

There is another issue open and I'm pretty certain this is the Caves and Cliffs Backport API, so I'm going to take this as a duplicate of #612.

Do you have Caves and Cliffs backport installed? I have seen similar screenshots reported relating to that mod (if so there is already an issue open #612).

Do you have Caves and Cliffs backport installed? I have seen similar screenshots reported relating to that mod (if so there is already an issue open #612).

I saw that claim as well and I did, at the time of capture, have it installed but I removed it, generated a new world, and the issue is still there. I also tried to find the BGcore mod that it was supposedly dependent on that caused the issue but it wasn't there.

I believe the issue is caused by their API but according to a changelog it should be fixed. If it's not fixed by removing/updating then please send a mod list.