Hello Dave,
Thank you for posting question on Microsoft Windows Forum!
Based on the issue description. Well! The reason this happens is probably of that Global Admin rights are a cloud-level permission (Entra ID), while Local Admin rights are a device-level permission. Being a Global Admin does not always automatically inject you into the local "Administrators" group, especially if the device was "Registered" rather than "Joined" or if MDM policies are active.
To check the "Join" Status by opening Command Prompt. Type dsregcmd /status and look for AzureAdJoined : YES. If it says NO, you are only "Registered," and you will not get admin rights automatically.
If you can manage Entra (Azure AD) or you are allowed to change tenant device settings. Sign in to the Microsoft Entra admin center (Entra ID / Azure AD). Entra ID → Devices → Device settings → Manage Additional local administrators on all Microsoft Entra joined devices → Add assignments → add your user (or a group) or assign the “Microsoft Entra Joined Device Local Administrator” role to a group/account. Save. Wait a few minutes and on the device sign out/sign in (or run a sync). The user should be added to the local Administrators group. For more information https://learn.microsoft.com/en-us/entra/identity/devices/assign-local-admin
If your organization uses Microsoft Intune or Endpoint Manager to control local admins. You can consult this article for more information https://learn.microsoft.com/en-us/intune/intune-service/protect/endpoint-security-policy
Hope the above information is helpful! If it is. Free feel to hit "Accepted" for benefitting others in community having the same issue too.