(I had reported this previously as well, but it probably deserves a separate task.)
At zh-yue, setting is Remember my last editor, I launch VE, then start hitting Special:Random.
Click on Edit, VE launches, hit Special:Random again (without closing VE).
Sometimes it takes a few attempts, sometimes it take multiple tries, but in the end I'll always land on some article which will show me "Edit source" rather than Edit (and I obviously haven't switched).
I do not see a warning message.
The above is how I manage to reproduce the behaviour. I have no idea if this also occurs while editing more "regularly".
As of July 2017, looks like the same thing, or a very similar one, is reported at https://www.mediawiki.org/wiki/Topic:Ttle85qn8vj8o9ml for pa.wp .
However, because not many details are provided, and I can't reproduce any of that, I don't know that we can diagnose anything here.