Do you happen to have a diary for this grow? Perhaps it's hidden? This is such an odd symptom we'd really need to see the diary if it's available... Let me just say that, if it's not a natural fade of the plant, whatever it is has been caused by a pH that's out of a good range.... If you're growing in soil, your pH must be between 6.0 and 6.5... if you're in hydro or DWC, that pH needs to be between 5.5 and 6.2... Out of that range, the nutrients needed by the plant will not be able to be absorbed...
If you don't have a diary, please put one together with as many weeks as you can recontruct... it doesn't have to be perfect but please try to get all of the information - auto or photo, how old, type of grow (soil? Hydro?), temps, humidity, pH, TDS, nutes and their amounts.. and then come back and ask your question again... This is really puzzling.....
Good luck