Firstly, nobody can be sure that these issues are a widespread problem in all iPhones or only in a few ones. There is no enough information to claim either way. Secondly, the first batches of any product mass-produced will have problems or issues, and Apple is not an exception to this.
Based on what I have seen so far, there is an inconsistency with the white balance in the 12 series. How do I know this? My first 12 Pro Max was very warm or yellow with washed-out colors compared to my second. My friend's 12 Pro was worst compared with my first one. At Costco, I saw a 12 Pro Max almost identical to my iPhone X in terms of white balance. The screen's clarity and depth were noticeable, not only for me but also for some employees at Costco, who compared my 12 Pro Max with the unit on display.
I also had the opportunity to compare one of my employee's 12 Pro Max, graphite-colored, and his screen was almost identical to my iPhone X. his unit was purchased on launch day first hour. My friend's wife recently got a new 12 Pro Max Gold, and her screen was almost near my iPhone X but not as yellow as my second 12 Pro Max.
The raised black or flickering blacks issue was present on my first 12 Pro Max. The second only exhibited raised blacks when the brightness was set around 10% to 15%. On my friend's iPhone 12 Pro, the raised black, green halo and super greenish grays can be seen even in daylight.
So, what do we have? There is an inconsistency problem with the new iPhones 12 Pro displays for sure, and I highly doubt this is intentional by Apple, even if they intended a more warm color profile for this version. How widespread is this issue? Unknown. Yes, Apple intended to calibrate the display near the D65 standard but looking at professionally calibrated monitors using this standard, those are closer to my iPhone X than my iPhone 12 Pro Max.
Can Apple fix this with an update? I think the raised black can be fixed but not the warmer profile. The discrepancy between iPhones makes it impossible.
There is not enough information to determine a correlation between the serial number and production weeks with good or bad displays. It looks completely random. In my case, I decided to return both 12 Pro Max and get a refund until Apple sort this. I won't pay for an inferior device to my previous one in such an essential component as the display. But I know many people who don't care or won't notice this issue if they have it, making it more difficult to know for sure the ratio between good and bad displays.
So if anyone claims that all iPhones 12 have this issue or that only an insignificant fraction have it, well, both are wrong and biased.
BTW, all my comparisons were with TT, NS, AB, and color filters off.