A couple of years ago a bitbucket-hosted project merged a pull request we submitted, and we were @'ed in the commit message:

https://bitbucket.org/LaNMaSteR53/recon-ng/commits/9fdc4d7e7cd4f255e3ebea9552d242faae182841

That generated a notification email as expected; "Email me about... Someone mentioning me" is enabled.

Ever since then, apparently every time someone forks the LaNMaSteR53/recon-ng repo, we get another notification from that old 2017-04-17 commit 9fdc4d7.

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.