Spent Rodinal Surprise

Recently I had a scare souping Arista Pro 50 (Pan F+ ?) in Rodinal 1:50. The spent developer dumped out clear. I initially thought that I might have accidently started with fixer, as every other time spent rodinal came out distinctly purple-ish. Negatives turned out fine (enough) Is this someting peculiar to the film/developer combo?
TIA
Charles

If you don't use a pre-soak, then the purple you see when pouring out developer is the anti-halation dyes. Different films have differently colored dyes, and few have none at all. Washing off the AH dyes is a function of simply soaking the film in any liquid, not just developing it.