I have to say that the difference between Stock and 3rd-party could simply be that the 3rd-party apps are addening sharpening.
However, the quality seems to have taken a big hit since 14 Pro, which easiliy has the best results here. It's almost as if they started using other lens elements of inferior quality. 15 & 16 looks out-of-focus compared to 14 Pro.
edit: Actually, it looks like there is something bad going on in the image process on the 15 & 16. They've these weird artefacts. Probably something that didn't exist on the 14 Pro.
Either way I'd return it, sorry. The camera is a huge deal if you buy these phones.
Try some other mode and see if it's still present there?