I use Newsblur’s app version 9.1 on iOS 13.2. I can reliably reproduce on both my iPad and iPhone that when I start the app anew and try to mark posts as read, the pop-up seems as if it has been scrolled further up:
This makes tapping on “Mark everything as read” particularly difficult. However, subsequent attempts work as intended. This only occurs the first time after the app has been properly closed and opened.