Last-viewed pages are saved in a cookie. So if I set the filter on the activity stream to “Show Blog Comments”, a cookie to that effect is stored in my browser. Next time I load the page on the same browser, “Show Blog Comments” is shown.
In global.js, I believe the function that handles this is bp_init_activity(). If you comment the call to this function out (around line 15 of global.js), it should stop the cookie from overriding your default settings.
