I can’t replicate this here on Meta (DiscourseHub, iPhone 13 pro, iOS 15.6.1). I checked with both a topic and a reply. Are you up-to-date with upgrades?
Actually, I think maybe I can now. I think it happens in replies rather than topic OPs, and also happens in Safari.
- Open a reply in edit mode
- Click on
edit reason icon while keyboard is open - Composer jumps to bottom and edit text box is no longer visible on screen
- Using normal swipe down returns focus to main text area
- Selecting edit text box again jumps back to bottom
(Closing the keyboard or using the side scroll before selecting the edit text box can be effective as a workaround, but is not intuitive)