Unable to unsubscribe from "when someone I know joins, with a key I might want" emails

When I uncheck "when someone I know joins, with a key I might want" on the notifications page, then click save, I get a green "Update succeeded." message.

However, when I refresh the page "when someone I know joins, with a key I might want" is back to being checked and I continue to receive emails (I've done this a couple of times now, which is why I did further investigation).

It looks like "Unsubscribe from all email" has the same effect.

I'm using Chrome 41.0.2272.118 m (64-bit) on Win7 x64.

Start at the notifications page and reproduce the unsubscribe and save flow described in the report, then inspect what happens after refreshing the page. The work is done when the unchecked option remains disabled after reload and the corresponding emails stop being sent.

Written by the indexing model from the issue text.