I wonder if this is related to T134593: Intermittent corruptions on saves, likely related to data-parsoid mis-matches.
I had mentioned there that we could probably add some heuristics to detect these data-parsoid mismatch scenarios ... maybe worth reviving that idea.
@Trizek-WMF Not much time to check what's happening, but you can use CW lists to have a quick glance at potential candidates.
For example, the list for frwiki
which immediately gives the following example
CW also generates lists for other wikis, but I don't know for which there are people actively fixing them.
gives several hits
A user told me he had done a copy-paste from a page in VE edit mode to another one in VE edit mode. Unfortunately, his edit was done on June, 21st, so the system has been updated on itwiki and I can't reproduce the issue.
It's possible that this will now be fixed by T146054: VE should strip element ids from HTML that it generates when wikitext is pasted (which will go into production from 25 October). Consequently, let's come back to this in a month and see if it's re-occurred after that.