Given the limited statistics we have from this thread, and projecting to the "bigger picture" that KB5008212 has probably installed successfully on significantly more computers than on which it failed, leads me to suspect that when KB5008212 fails to install successfully on a computer, there is an issue with that computer. This is probably the reason that MS hasn't responded. In other words, there may not be anything "wonky" with this update; rather, there may be something "wonky" with the computers on which it fails to install.
Firstly, thank you for your previous post on the fix. Using "Update Assistant" from this link, Download Windows 10 (microsoft.com). I didn't really do anything else other than that.
There was one difference I noticed afterwards that may (or may not) help others with this issue. Originally, I had tried to manually download the update from Microsoft's Update Catalog. The update that my computer was trying to use was "2021-12 Cumulative Update for Windows 10 Version 20H2 for x64-based Systems (KB5008212)". After using the Update Assistant and letting the computer update, my Windows is now on Version 21H2.