One editor reports that sometimes, it switches from the new wikitext mode into the visual mode.
Reported by @MotokoCK, who is using Chrome on macOS Sierra 10.12 at https://www.mediawiki.org/wiki/Topic:Thnu54tj48r51n0z He primarily edits at the Korean Wikipedia, which uses the Single Edit Tab.
See contributions here: https://ko.wikipedia.org/wiki/특수:기여/Motoko_C._K. There's no indication that this user has ever switched into visual mode (no "switched" tags), but sometimes the visual mode is opening anway.