My first thought is how long are you leaving the lights on? if more then 8 hours a day try reducing the duration to 8 hrs and see how that goes.
Not sure why your adding diy CO2 and excel the CO2 should be enough according to what your drop checker is reading. CO2 indicator = drop checker.
You might want to get a small syringe and dose the affected leaves directly with the excel to kill off the BBA.
As for why it keeps coming back I wish I knew.