What is the ruling if a worshipper begins to make the takbir while the Imam is already in the bowing position (ruku') and is unsure if they caught the minimum required portion of the bowing with the Imam before the Imam rose?

Primary text

If the worshipper is unsure whether they reached the minimally sufficient portion of the bowing with the Imam, that unit of prayer is not counted as having been performed with the Imam. The worshipper must then pray four units of Dhuhr prayer instead. This is the unanimous opinion because the default presumption is that the required action was not attained with the Imam.