FYI: DiscussionTools makes talk page sections uncollapsible on mobile has been reported in phab:T280433.Xaosflux (talk) 09:28, 17 April 2021 (UTC)
Topic on Talk:Talk pages project
To add, sections in project space have become uncollapsible too. Large pages like en:Wikipedia:Administrators' noticeboard/Incidents are very difficult to navigate in mobile view because of this.
On the theory that people who encountered this unfortunate bug may be interested in the mobile site:
The Reply tool may become available for editors using the mobile site some time during July. (No promises.) It looks and works approximately the same as the desktop version, except that when you're typing, the box is full-width, so it will look like it's going to reply without any indentation, but it will be in the correct (horizontal) place when you post the comment.