I’m having the same problem, and just like you, it’s only recently.
The first time I send the user to /oauth/authenticate with force_login = true, it uses the currently authorized account and THEN logs the Twitter account out (unexpected).
If I try to go through authentication again, I’ll have to log back in (expected).
I am also sometimes getting a “bad access token” error message when using force_login = true. I thought it may have been just me, but I actually encountered the same error when authorizing with this site! See the screenshot here.
Again, this is only recently this has been happening. I don’t see any changes referenced in the docs; what’s up?