This is an issue with the Safari Technology Preview. I've reported it and it has been fixed in ToT WebKit, which means it should show up in a Preview release eventually. In the mean time I was able to login using Safari and then that login made its way to the Preview.
Thanks, Jon. I also noticed that switching my cookies setting from "Allow from current website only" to "Allow from websites I visit" helped with this and another issue. Do you know if it's related? A link to the bug would be appreciated.
Note, the reason for this is that somehow service workers in the preview are intercepting off site redirects. Service workers are a brand new feature in the preview.