Leaf Lighting issue

Is it possible for you to provide a video?

https://youtu.be/tvYFKVSd4ZY

This looks like low mipmapping. What's your mipmap level in Options -> Video Settings?

Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (minecraft/crash-reports/crash-<DATE>-client.txt) here.

NeunEinser, this disappears by setting Graphics to Fancy.
Other than that, the mipmap level is at its highest (4), and odd lighting lines appear on the tree leaves.

Was there any difference between having it on 4 and turned off?

There was no difference between off and 4. It was the same, odd leaf lighting.

As already requested: force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (minecraft/crash-reports/crash-<DATE>-client.txt) here.

A crash has been attached, this is still present in 1.15 pre2.

Update your graphics driver and see if the issue persists with modern driver. Current driver is almost 5 years old and might contain bugs that are relevant to this.