After more than two weeks, I think that it is time to sum up the situation: the "flashing Apple logo" issue described by Blue-per does not occur when we unbox and boot the iPhone 12 (mini)/12 Pro (Max) for the first time, it occurs when the iPhone is already set up.
In most cases the phone performs normally after it boots up. Some users found hardware problems when using their new iPhone (phone needing to be connected to power in order to boot up, screen burn-in, battery charges very slowly, camera not taking HDR photos, etc.).
In my opinion those problems are not related with the flashing logo at startup, because they are very different from each other and they are isolated cases. They are just coincidences.
That does not mean that the flashing logo is not an issue: there are iPhone 12/12 Pro which boots with the (classic) steady Apple logo and some iPhone 12/12 Pro that do not, so it is obviously not a normal thing. It is not a "new feature" or "expected behavior".
After reading other answers on the forum, we can deduce that the issue does not occur on iPhone 12/12 Pro that were shipped with iOS 14.1 or with iOS 14.2 pre-installed, it (mainly) occurs on iPhone 12/12 Pro shipped in January with iOS 14.3 pre-installed. As a matter of fact a lot of people returned the device during the last weeks and got a new one with the same problem.
Now, we suppose (and hope) that the issue is part of iOS 14.3 and not in the hardware of the phone. In this case Apple should be able to fix it with an update, maybe already with a sub-dot-release (iOS 14.4.X) or later with iOS 14.5. There are some comments in favor of that supposition too, where people noticed that issue after updating from iOS 14.2 to iOS 14.3.
On the other hand, it is strange that (most) iPhones who updated the operating system from 14.2 to 14.3 did not experience the issue. And it is even stranger that I read a reply from a user who encountered the problem already on iOS 14.2. Those facts contradict our supposition above, and maybe the flashing logo has nothing to do with iOS 14.3.
I am sure a lot of people have this problem too; they do not notice it, because nowadays we do not shutdown and restart our phones very often. And even if they do that, they (probably) don't keep looking at the boot screen for issues.
It is very hard for us to guess if it is a software or a hardware problem. Only Apple can give us an official answer, and in order to solve the problem as soon as possible I ask you to continue upvoting this and other similar posts (clicking on "I have this question too"), commenting if you have some news from Apple and reporting the case to the Apple Support.