This was what happened to me too. Started feeding more which led to high PO4 but still had the same burnt tips. The mistake I had was not testing for NO3 as my NO3 was bottoming out. Dosing NO3 did help and my SPS are doing better now.
Would also recommend to stop all forms of dosing (bacteria/carbon/amino acids) and just rely on good old water changes for the next 2 months.