A couple of years ago a bitbucket-hosted project merged a pull request we submitted, and we were @'ed in the commit message:
That generated a notification email as expected; "Email me about... Someone mentioning me" is enabled.
I thought this might be a FAQ but didn't find anything from searching for a few obvious keywords.
It really doesn't make sense to regurgitate old notifications every time a project is forked, it's wasteful of bitbucket.org resources and it spams your users/contributors to hosted projects.