Just for interest, in case it affects anyone else, we also recently found hybrid Azure devices (domain-joined Win 10 machines) in a non-MDM/Intune environment that were showing as non-compliant. All joined since the 30th of November. Previously-joined devices were showing as N/A as expected.
After locating the never-used Policy page in Azure Portal (https://portal.azure.com/#blade/Microsoft_Azure_Security/SecurityMenuBlade/), I found the ASC Default policy (part of Defender for Cloud) has somehow become enabled on our main subscription. It seems that some Defender for Cloud trial licenses got assigned and resources in the subscription came into scope.
What exactly happened with this feature and policy being enabled is still unclear, but for us, it is this ASC Default policy applied in the subscription that is causing the most recently-joined machines to be marked as "non-compliant".
Nothing can possibly be compliant with this policy in our environment, since it hasn't been designed for it - we don't use Defender for Cloud.
I note that last month "Governance Rules at Scale" was released into Preview for Defender for Cloud, where bulk rules can be configured at subscription, connectors or various scopes. I wonder if these changes may have suddenly "activated" this policy, at least for us.