Ref T13121. When you connect to a host with SSH, don't already know the host key, and don't have strict host key checking, it prints "Permanently adding host X to known hosts". This is super un-useful.
In a perfect world, we'd probably always have strict host key checking, but this is a significant barrier to configuration/setup and I think not hugely important (MITM attacks against SSH hosts are hard/rare and probably not hugely valuable). I'd imagine a more realistic long term approach is likely optional host key checking.
For now, try using LogLevel=ERROR instead of LogLevel=quiet to suppress this error. This should be strictly better (since at least some messages we want to see are ERROR or better), although it may not be perfect (there may be other INFO messages we would still like to see).