On german wikipedia we have see this already, but it seems to be a browser issue, not a issue on save.
When open the old version[1] in some FireFox configuration the replacing is seeing in the textarea (But than the user must edit the whole page and not only one section).

[1] https://hu.wikipedia.org/?action=edit&oldid=16336110&section=11

This seems to be a section edit though. Also, anecdotally, such changes have become more frequent recently.

In T107679#1503941, @Tgr wrote:

This seems to be a section edit though.

If there was an edit conflict that had to be resolved manually (which doesn't seem unlikely), than actually the whole page was edited in the browser, even though the summary looks like a section edit.