When creating a new article, each template has a button in the top right that should allow you to create the article in the old editor. (I find this is useful when creating several articles at once, as too many new editor tabs lag *hard*). Clicking this button does nothing different, making articles in the new editor as normal.
It is also possible that it's creating articles twice, and opening one with the new editor - I just discovered that for each of the 10 articles I made earlier today, I had a blank one of that article template listed as UNTITLED.
Just made 3 dummy articles to confirm - one by pressing the old editor button, one clicking specifically the create button, and one I forget where I clicked - all 3 spawned UNTITLED duplicates. Not sure if this is a related problem or deserves its own bug report.
I am having similar issues. From the create a new article template I hover and say to create it using the old editor but it still doesn't work. When in Visual editor, articles don't save and/or I get constant messages to upgrade my membership. When I switch to Euclid everything works fine. My settings are to use Euclid as my editor but it doesn't change anything.