The update installed fine on two of my Win 10 computers but not the third. There is nothing wrong with that system other than the fact that it is running Windows 10. Like I said other people are reporting the same issue so it looks like it's just another wonky Windows update.
I, too, am having a problem installing KB5008212, although a somewhat different one - I must have tried to install this update about half-a-dozen times by now without success. I have a Dell desktop running Windows 10, version 21H1, 32-bit. The first couple times I tried to install the update, upon Restart the computer got stuck on the black screen that follows right after the initial blue Windows logo with the little white dots going around in a circle. On that black screen the little white dots were going around in a circle endlessly, and I mean endlessly, without further progress. I even left the computer on over night, and in the morning those little white dots were still going around in a circle.
On one of the later attempts something must have gone on while I was away from the computer, the computer booted up, and the update history showed (and still does) that the update was successfully installed on 12/21/21, yet, when I checked the list of installed updates via the Control Panel, KB5008212 was not one of the listed updates. Sure enough, the next day there was a new notice that KB5008212 has downloaded, installed, and Restart was required to complete the installation. I thought that was weird, but I hit the Restart anyway and let the computer go through the process. Unfortunately, it once again got stuck on the black screen with the little white dots going around in a circle.
After several unsuccessful attempts, today I decided to manually power off the computer and restart it several times till it informed me that it is diagnosing the problem and attempting to fix it. I let it run its course and, sure enough, it booted up normally, though it took quite a while longer that usual. When I checked the update history, it still showed that KB5008212 was successfully installed on 12/21/21, yet it was still missing from the list of updates obtained via the Control Panel.
I tried a whole slew of "tricks" that were suggested by various resources on the Internet, but none fixed the issue. The only thing that I have not yet tried when the KB5008212 is still awaiting Restart is a clean boot, because I'm not sure this will fix the issue. So, for now, I decided to pause updates for 7 days hoping to find a solution that will fix this problem.