Since {T188833}, changes to alien nodes are shown in a simple way in visual diffs. But in some cases, they show even if they weren't changed, while sometimes they are omitted as expected.
https://de.wikipedia.org/w/index.php?title=Internationale_Mathematik-Olympiade&action=history has many examples of this behaviour. The article has 3 `blockquote`s, which aren't changed in most of the edits, but sometimes show up as both deleted and inserted again, with no apparent logic.
E.g. https://de.wikipedia.org/w/index.php?title=Internationale_Mathematik-Olympiade&type=revision&diff=177245738&oldid=177216255&diffmode=visual shows one of the them. https://de.wikipedia.org/w/index.php?title=Internationale_Mathematik-Olympiade&diff=prev&oldid=176981978&diffmode=visual shows none, https://de.wikipedia.org/w/index.php?title=Internationale_Mathematik-Olympiade&diff=prev&oldid=167603307&diffmode=visual all three. At least, that's the way these diffs currently behave for me, I'm not entirely sure, but the behaviour doesn't even seem to be stable for one diff.