Hello @cat-grep
Initially I can recommend you the tool SetupDiag that can help determine what may be the cause of blocking the upgrade:
https://learn.microsoft.com/en-us/windows/deployment/upgrade/setupdiag
On the other hand, to workaround the version limitation you could set up a WSUS server and deploy the feature update for these computers up to 1909 and then let Autopilot/MDM do the reest. Here is a comprehensive guide to the WSUS Setup for this kind of purpose:
https://learn.microsoft.com/en-us/windows/deployment/update/waas-manage-updates-wsus
Hope this helps with your query,
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