Problems with fan leaves

Hi guys, so I have an auto that's about five weeks old barely a week into flowering
And some of the fan leaves are starting to yellow and basically die.
I feel like it's too early for that to be happening
I use biobizz I follow the feeding schedule
I pH my water before every feed
So I have no idea why this is happening
Any advice would be greatly appreciated

here is the contrast
It just gets more and more pale and dies :/

no biggy skip 2-3 feeds

continue watering a per usual

if continues for the next month

I'd potup and go half feeds

good luck