While the bug will be triggered in this scenario, I found that it was not necessary to switch back to the source editor to trigger it. But it is necessary to "publish" the edit to observe that all other text has been erased from the page.

VisualEditor is not supposed to support section=new. This looks like the same issue as T223369, except that something has changed in the editor switching code since I reported that in 2019, and instead of losing your input (and opening the current version of the page for editing), it keeps it (and replaces the whole page).

Actually, that bug report was just about the new wikitext editor, while this is happening in the old wikitext editor, so it might be a new problem.

I'm removing this from the current workboard after @matmarex helpfully explained that this issue is confined to instances where people are:

1. Starting new discussions without using DiscussionTools
2. In a namespace where discussions talk place and the VisualEditor is available

"1)" and "2)" lead me to think this issue is less urgent than I originally understood it to be.