You should be prompted when opening after updating to the latest version.

Yeah I agree. I think the filtering system as a whole could use some reworking. Hopefully now that there is a growing active community to highlight the issues, the devs will be able to pinpoint and optimize the algorithms. Even Hot has issues with randomly throwing loads of 1 year old posts on top, or seemingly treating any new post as “hot”.

Your default filter can be set in your profile settings.