Example of your "cannot explain why". My box has an Intel HD630 iGPU on the board, though I never use it, I use the 1080Ti. But, because it;s there, MS has me on a block.
Attachment 281424
Variable refresh rate not working as expected on devices with Intel iGPU
Intel and Microsoft have found incompatibility issues when using a monitor with Variable refresh rate (VRR) plugged into an Intel integrated graphics processing unit (iGPU) display adapter and Windows 10, version 2004 (the Windows 10 May 2020 Update). Enabling VRR on affected devices will not enable the VRR for games using Direct X 9.
To safeguard your update experience, we have applied a compatibility hold on Windows 10 devices with affected drivers or firmware from being offered Windows 10, version 2004.
Affected platforms:
Client: Windows 10, version 2004
Next steps: Microsoft and Intel are working on a resolution and will provide an update in an upcoming release.
Note We recommend that you do not attempt to manually update using the Update now button or the Media Creation Tool until this issue has been resolved.
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Because I had used WU then the ISO on top of it, I compounded the error, I think. I tried re-doing with ISO and it got worse. MCT froze, Update assistant froze, My Check for Updates looks like this now.