We were able to resolve this issue by updating Windows Defender to version 4.18.2110.6. Once this was in effect all systems switched over to using AMP as the primary AV provider and disabled Windows Defender. We did not have to make any registry ch...
We're experiencing a similar situation with our Windows 10 workstations; AMP is not registering itself as the primary av provider thus Windows Defender is not getting disabled. Did your issue resolved, if so could you share?