How to re-allow apps in Health?

I'm using the app Pillow v3.3 for sleep analysis and have been able to see my heart rate in the app's overview for each night.

Since I recently moved from iPhone 7 to iPhone 8 via a backup, the apps which are adding data to the Health app can no longer access data from other apps within Health. In the case of Pillow, no heart rate data is shown, not even the previous available data.

I offloaded Pillow a few times, to keep the recorded data of 160+ nights. Unfortunately, no change has happened.

In Health, if I click on Vitals > Heart Rate > Data Sources & Access, Pillow should be listed in the section Apps Allowed To Read Data - but it isn't. If I check in Settings > Privacy > Health > Pillow, I can't choose Allow because there is nothing to choose, the screen is empty.

Searching here in Communities, I found a similar posting which indicates that apps connected to Health loose that connection data and will not transfer the connection data when transferring to a new iPhone.