If you go to the main page on the website and click "mark all posts on the site as read", then go back to the forums, all threads become unread.

This is known issue. It's been reported many months ago:

https://handmade.network/forums/feedback/t/8051-bug_mark_all_posts_on_site_as_read_link_doesn%2527t_work

https://discord.com/channels/239737791225790464/601850747872870401/891040856235204629

This should be fixed now.

Thank you! It works now for me.

Thanks.