About 3 weeks after flipping the plant finishes it's stretch phase and no longer needs as much Nitrogen.
Your FF nutrients automatically cut back on the Nitrogen and pick up the Phosphorous and Potassium levels for flowering. Sometimes the plant and the solution are not on the same schedule and the plant starts stripping
leaves.
I don't see any signs of Calcium or Magnesium issues.
Your plant is beginning to show her winter colours. Stripping old leaves of nutrients and dropping them.
I know it does not look very nice but she is just doing what plants do at this stage of there life. But, your next plant could stay vivid green right to the end. Ya just learn to deal with the plant's requirements and not worry so much.
Nothing to worry about here bud. She looks great! Very nice job so far!
Don't try and fix her k.