On Rails 6 I’ve followed the install directions for webpacker. I’ve exported Turbo and can call Turbo.visit, but it always does a full-page replacement. Turbo.supported returns “undefined” in stimulus controllers and in web console (which would lead to the full page replacement I’m assuming). I have a frame working as expected. Tested on Firefox 83 and Safari 14.
@jinkies can you share demo app with the issue? I can take a 
ps
If you use webpacker, executing bin/rails turbo:install should “properly” setup turbo with rails webpacker.