There is a problem. I have 4 computers. On the first three, from Windows update, I did the KB updates KB5003637, KB5003254 .NET framework and cumulative 20H2 updates first. When I did that, the option to update to 21H1 disappeared seemingly permanently.
So on my fourth computer I did the 21H1 update first, and that went fairly smoothly, only took a few minutes, and then later the KB updates which took a while.
So then on one of the first computers I used the Windows Upgrade tool to update to the 21H1, since it was no longer available through Windows update, even after running the update troubleshooter. It started to install normally, then took hours to finish the installing section. Then when it went to "restart" for install, it gives you the dreaded "undoing changes made to your computer" and my computer then shut down unexpectedly when it shouldn't have. So the 21H1 install failed, and I still don't have an option in "Windows Update" to do it.
What the heck guys?? Why do they roll multiple update options out when they haven't even tested to see if they are compatible??? Do the programmers even test this stuff??? Now I have no idea what "corruptions" have occured.