Got the camera this morning and mounted it. But when trying to use it, only the front camera works, no flipping working. Dismounted it and put back the “old” (12MP), which works greatly.
You could try to clear the cache of your camera app for a start.
Open System Settings → Apps → select Google Camera (or any other third camera app you use) → Storage → Clear Data .
Maybe this helps and the app is “taking a fresh look” for a camera?
Well, as there are quite a few reports in this forum, that the exchange of the camera went flawless, it seems, you might have had bad luck and received a faulty camera.
If you know someone else with a FP3, you could try to exchange modules for testing; or maybe one of the #fairphoneangels lives in your vicinity?
In any case, you should contact support and maybe you will get a new module in exchange; maybe even faster, since you are inside the 14 day return period.