Interesting about Foma, I've never devved it but was thinking of getting some when my Chroma arrives (for comparison purposes).
I think the effect on contrast might have been related to what
@steveo_mcg suggests, but old posts in AP suggest the other way round. Yes, the stop bath does stop the development immediately as it doesn't work in an acidic environment. Using water for stop just dilutes the developer, which carries on working to a lesser extent:
"As far as developing times go, once you pass a certain minimum time, the shadows in a negative don't gain any extra density; it's the highlights and midtones that get darker. This translates to increased contrast. If you have access to an older edition of Michael Langford's "Advanced Photography" (I think it's that book) there is a very interesting set of negatives iluustrating this point. This was cut from the current edition..."
Read more at
http://www.amateurphotographer.co.uk/forums/threads/stop-bath.23885/#R7e2yGc3BKXEiEHY.99
The other thing O noticed when reading around was that they weren't just talking about adding a bit of water, they seemed to suggest lots of agitation, or multiple washes using the Ilford method.
BTW several commenters I found suggested that the stop bath improved reproducibility. But I suspect this was mainly about prints.