I've observed this occurs in Firefox 67 and Chrome 73 on macOS, although in a couple of cases it seemed like I had to log out and log in again for it to work.
- Enable the "New Wikitext Editor" beta feature
- Attempt to manually open the VisualEditor in namespace where it's disabled by adding the query parameter veaction=edit to the URL (for example, navigating to https://en.wikipedia.org/wiki/Wikipedia:Requested_articles?veaction=edit)
- Observe that the 2017 wikitext editor has opened, but the text area contains the page's raw Parsoid HTML rather than the wikitext. For example: