The lab says fixation will lead to the dyes to leak through the cells. I do think the assumption is the fact that fixation may possibly problems mobile membranes. I did find a paper which describes fixation utilizing methanol or acetic acid, immediately after staining that has a DCFDA spinoff. https://erickpdpyh.theblogfairy.com/27653708/not-known-factual-statements-about-2-fdck-hcl