Yeah, I initially thought this was a problem with the debian/testing64 box, since debian/bullseye64 running on a bullseye host comes up cleanly. The problem lies with the Debian vagrant version and its way of invoking Net::SSH.
I think that reassigning this report is the right course of action, and your summation sounds right too. Pointing the FAQ here would help users trying testing on stable via vagrant out a lot. It looks like there's a nice upstream patch (containing a monkeypatch?) that can be applied to the version of vagrant in stable too: https://github.com/hashicorp/vagrant/pull/12298/files. It's targeted at exactly the Net::SSH 6.1.0 that's in Debian stable right now. The vagrant maintainers will have the option of backporting the patch or releasing a newer vagrant in the next Debian 11 point release: up to them. I'm happy to test things out ;) - Andrew
