Well, still, thank you for your help! I will continue searching for what's happening, so if I find something, I'll post it here
So far I figured out that the update doesn't affect the whole chunk, several layers of grass or leaves can be different colors at the same time
I am sorry, but I don't know why it doesn't work. The client receives the chunk data packet and ithas the correct biomes in it. It then decides to re-render the chunk. I don't know why it renders with the wrong colors.