The lab suggests fixation will bring about the dyes to leak through the cells. I do think the belief is the fact that fixation may well damage cell membranes. I did discover a paper which describes fixation using methanol or acetic acid, just after staining by using a DCFDA spinoff. https://2-fdck-hcl16034.amoblog.com/little-known-facts-about-2-fdck-hcl-49832334