When any user resets password, all guest sessions got purged from the sessions table, so all  [guest] online statistics gets nulled.

The cause is the following.

The security fix https://github.com/phpbb/phpbb/commit/827c135cc9b0b187a61a2d9e0f64592cce7418ff

has added $this->user->reset_login_keys() function call without parameters. A user is not logged in while resetting password, hence guest user_id is used to clear all user's sessions.