You can set app limits for individual days of the week and vary the time allowed for an app, as demonstrated in the image below.

You're right, there's no point in having two limits set for the same app in Screen Time if the shorter limit takes effect first. Here's what's happening:
- Screen Time applies the strictest time limit you've set for an app.
- In your case, even though you set a 1-hour limit later, the 30-minute limit you set first is the one that's enforced.
See the pic below (1 Hour 35 Mins and 1 Hour 40 Mins) The App limit exceeded after 1 hour and 35 Mins

There are two ways to address this:
- Remove the extra limit: Go back to Screen Time settings and remove the 1-hour limit you set for Instagram. This ensures only the 30-minute limit is active.
- Set the desired limit: If you want a 1-hour limit, remove the 30-minute limit and set a new one-hour limit for 1 hour.
Following these steps ensures that only the desired time limit applies to your Instagram usage.