In Azure, in CentralUSEUAP, when creating an OCP cluster (applies to ARO too), worker machine fail at being created. Looking at error, message, it seems that the underlying Availability Set creation fails with error 
AvailabilitySet "<somethingsomething>" with platformFaultDomainCount = 1 can only support platformUpdateDomainCount = 1

This error echoes some of the things in https://issues.redhat.com/browse/OCPBUGS-45663. The way I understand the MAPI code 
https://github.com/openshift/machine-api-provider-azure/blob/5a6516188d4ec33734e1a069da2acc7a469657dc/pkg/cloud/azure/services/availabilitysets/availabilitysets.go#L48

is that to fix OCPBUGS-45663, the platformFaultDomainCount is now computed dynamically to 1 for that special region. But the platformUpdateDomainCount is hardcoded to 5, which sounds to be incompatible with platformUpdateDomainCount set to 1 (apparently, Azure seems to expect platformUpdateDomainCount to be only 1 in that case). 

    observed 4.16, 4.17, 4.18

systematic    

    1. Create an OCP cluster on Azure (or an ARO cluster) with any of the versions that contains the fix for https://issues.redhat.com/browse/OCPBUGS-45663  in CentralusEUAP
    2. Worker Machine creation fail.
    3.
    

    MAPI does not create the underlying Worker VM, error appears about "AvailabilitySet "<somethingsomething>" with platformFaultDomainCount = 1 can only support platformUpdateDomainCount = 1"

Worker VM are created and machine goes running    

    This error echoes some of the things in https://issues.redhat.com/browse/OCPBUGS-45663. The way I understand the MAPI code 
https://github.com/openshift/machine-api-provider-azure/blob/5a6516188d4ec33734e1a069da2acc7a469657dc/pkg/cloud/azure/services/availabilitysets/availabilitysets.go#L48

is that to fix OCPBUGS-45663, the platformFaultDomainCount is now computed dynamically to 1 for that special region. But the platformUpdateDomainCount is hardcoded to 5, which sounds to be incompatible with platformUpdateDomainCount set to 1 (apparently, Azure seems to expect platformUpdateDomainCount to be only 1 in that case). 

I am not certain this is something that changed recently on Azure side or if the incompatibility between those two paramaters has always been there.