The sign-in page remembers how you signed in last time and opens on that method. Where that method was a password, the page was doing something wasteful to get there: our server sent one version of the form, your browser decided it wanted the other one, and rather than adjusting it discarded everything it had been sent and built the page again from scratch.
What you saw was a flicker as the page settled, and a short spell in which the fields were on screen but not yet ready to accept what you typed — on the one page where you are most likely to start typing immediately. Nothing was ever lost or mistyped as a result, but the delay was real and it was avoidable.
The page now agrees with itself from the start. Your remembered method still applies, your email address is still filled in for you, and choosing the other method still works exactly as before.
Signing in with a magic link was never affected, because that is the method the page already opened on before it consulted your browser.