One plant gone like golden colour

That looks like the start of a sulfer deficiency. Suffer moves slowly through the plant so may take a few days to correct. Make sure your water is correct ph.

Exaclty the plants looks also healthy to me! But imagine one of your fingernails has torn a bit from your finger. U just clip that nail off, nothing wrong with your hand😎

Could it be you accidentally splashed nutrient water on it?

I have never seen yellowing from the inside( still a noob ). It could be "Variegation" which is either from a virus or genetics.

You say rot? I will chop but I want to know what it is to try avoid the issue again never seen anything like it on any deficiency charts as you see rest of my plant looks healthy

Chop chop. I clip that away. See if nothing else goes rot the next day