I was able to reproduce this my removing cookies while on the edit page. After clicking "submit", I am greeted by "you have to log in". The text is gone, and not even the back button brings it back.

This issue has lead to data loss several times in the last few weeks just on our Wiki. We are taking minutes of our meetings in the Wiki, and several of them have been lost because people's sessions expired during the meeting.

Phew, so I'm not hallucinating here :)

There are no intermediate proxies. I have been hitting F5 in the browser for all tests. I have no cache configured in LocalSettings - all I found is this commented-out line:

I cannot understand the assignment of priorities here. This bug is a completely silent footgun, destroying parts of the Wiki functionality for people that configure their server for strict security. It just took me >1h to debug why sometimes, nondeterministically, updates of templates are not properly propagated to the pages using these templates, and even calling "runJobs" later would not fix this. Turned out that for several months now, MediaWiki sent the job updates to the wrong place, got back an error, *logged that error* (well, if debugging was enabled), and still considered the job as being executed. I have a redirect from "http://" to "https://" set up, and MediaWiki was stuck on this without telling anybody.