Why YSK: It will start working faster if you give it a sec.
This is a bug in iOS Progressive Web Apps. Your scrolling gets locked to an element that is off the screen. Continuing to try to scroll while it’s in this locked state keeps it in the state longer. No interactions for one second will unlock it.
wefwef are tracking the issue here.
Another fix: Drag all comments (or posts) from left-to-right in an oscillating motion, and this seems to “unstuck” the vertical scrolling. I think what’s happening is that a given comment (or post) is getting stuck ~1 pixel off-center, so the app assumes you’re still focusing on the comment’s horizontal scroll pane (rather than the overall page’s vertical scroll pane).