The lab states fixation will bring about the dyes to leak with the cells. I feel the idea is the fact that fixation may damage cell membranes. I did look for a paper which describes fixation utilizing methanol or acetic acid, right after staining by using a DCFDA spinoff. DCFDA https://francisconllez.ziblogs.com/29479882/2-fdck-hcl-fundamentals-explained