Why does a model say "failed recently"?

That model did not answer on this device in the last 30 minutes, so the picker flags it. You can still choose it and still send — the mark clears the moment it replies successfully, or after 30 minutes. It is only ever a warning, because a failed reply is often a patchy connection rather than a broken model, and it applies to your device alone: nothing you see here changes that model for anyone else.