Apologies if this has already been reported, but I think this is separate from the issues in T357724 and T357770. The line-height changes and mobile font size setting only seem to apply to <p> elements. This leads to inconsistent appearance, most noticeable if Font size is set to anything other than "Small". Some examples:
en.wikipedia Main Page: "Today's featured article" blurb is larger than "In the news" and other lists
Lists in articles (example from https://en.m.wikipedia.org/wiki/Super_Bowl_LVIII#International)
Talk pages: Top level messages are larger than replies (example from https://en.m.wikipedia.org/wiki/Talk:Super_Bowl_LVIII)
Note this isn't an issue for the desktop font size setting beta feature, is it possible to use that approach on mobile instead?