In the mean time I tried devices in the store for their behaviour. That excluded a hardware defect on my device as both a 14 pro and a 14 pro max in the store showed the same issue. In fact other iphone models with dual camera showed the same issue but is was less present as the used lenses apparently have a shorter focus.
So it seems to be software. Apps showing the issue apparently do not (cannot?) use a library that uses the auto camera change.
Even *Apple's own* 'Code Scanner' app (Settings -> Control Center -> +) has the same issue.
I wonder whether Apple will fix only their own app(s) or come with a solution in a library that can be used by any 3rd party app!