From OTRS:

Yes, I’ve tried editing specific selections, but find this functionality too buggy to be of use at present. The forced appearance of the keyboard tends to hide my original selection, or the editing view scrolls up to the top of the (parent) section, meaning that I have to manually find my place in the subsection. I usually do this by searching for a keyword, but this method is slower.

I have to restate my request for a subsection edit button. This is more familiar to editors used to the desktop browser version, and would just work better within some of the more lengthy parent sections.

Also documented in https://phabricator.wikimedia.org/T179701.