The lab claims fixation will cause the dyes to leak from the cells. I think the assumption is that fixation could destruction cell membranes. I did look for a paper which describes fixation using methanol or acetic acid, after staining that has a DCFDA by-product. DCFDA may be the reagent https://damienhpwzd.blogpostie.com/50267315/a-review-of-2-fdck-hcl