In this edit I had used the "edit source" link next to the section header "Later career". I only changed the contents of one citation template. Before publishing the changes, I clicked on "Review your changes". The shown diff was completely wrong; it only highlighted some footnote number, but not the actual change I made.
This problem is reproducible if you follow the above steps.
I think this is fixed already? When I checked the visual diff, I got a red line showing:
Peace, William. "Head Nurses". Atrium. Retrieved 16 December 2015.
and a green line showing:
Peace, William (Winter 2014). "Head Nurses" (PDF). Atrium. No. 12 "Bad Girls". pp. 20–22. Retrieved 9 July 2020.
(both with the expected links for the title).
@Whatamidoing (WMF): No, the issue isn't fixed. If you click on this edit and then choose "Visual", you will indeed get a correct visual diff. But if you follow my steps (edit the section, change a reference, and review your changes before publishing the edit), the displayed diff will be wrong.
I had assumed that the diffs would match, but they don't. Phab:T260008 describes the problem. Please feel free to "Be Bold" and edit the description or add a comment to explain further, if you think anything is wrong/missing.