One way i know is remove all data of app (reset app) in information about application in settings, but if you are signed into boost and you remove all data of app and sign again it's possible that marked as read post will be still marked as read, but you can try it. Maybe it will work

Or you can remove checkbox for hide if read in post setting. or have you already done this?

Is it possible to reset posts marked as read? I thought clear read posts would do this, but it hid all the posts instead.

I've tried turning off "Mark as read" but when I turn it on again, all read posts are still marked. And the posts I accidentally cleared are still gone.

Reset "Mark as read"