This is not a manufacturing defect, but a design choice (i.e. cockup). I have seen iPhones from China, HK, and UK all with exactly the same flare problem. All display phone in UK Apple Store are also affected. Don't waste your time and try replacing/repairing your handsets as you will be repairing something that's not broken. It's working as designed, but the design is flawed.
If you are already the owner there are a few choices: either return it to Apple (if you are eligible) or learn to live with it, or switch to a different phone. Some say that all all comparable products have this issue. It's true, but the extent of the problem is nowhere near as bad as in iPhone. In Galaxy 10 series it's very subtle, while in iPhone 11 Pro it's **** obvious. You can see it yourself I the attached samples taken from exactly the same angle.
As for reducing the flare, there there isn't much you can do. Apple has officially confirmed to me that it's not a sensor issue and not a software issue, so don't wait for a magic fix. I was experimenting with manual settings and can report that different aperture and different ISO settings make no difference whatsoever.
My gut feeling is that they simply placed the protective glass too far from the camera. Would be interesting to see how it performs without the protective glass. Any volunteers?
In the mean time, we'll have to wait and hope that Apple will fix it in their next phone. One bit I really don't get: how could Apple engineers not notice it during the routine testing? Surely camera specialists should have put it through every possible scenario. Apple?