AzureMachinePool indefinitely scaling up and down the pool size

[MadJlzz]

/kind bug

What steps did you take and what happened:
[A clear and concise description of what the bug is.]

We're having a small problem with capz v1.17.1 and capi v1.8.4 using AzureMachinePool

Even though we've set the number of replicas for the pool to be 1, capz is infinitely updating the underlying vmss changing the size to 2 and reverting back to 1 a bit afterwards.

What did you expect to happen:

To keep the size of the vmss to 1, as stated in the spec.

Anything else you would like to add:

In production, we're using those version and didn't get this particular error:

capi-kubeadm-bootstrap-system       bootstrap-kubeadm       194d   BootstrapProvider        kubeadm       v1.7.2
capi-kubeadm-control-plane-system   control-plane-kubeadm   194d   ControlPlaneProvider     kubeadm       v1.7.2
capi-system                         cluster-api             194d   CoreProvider             cluster-api   v1.7.2
capz-system                         infrastructure-azure    194d   InfrastructureProvider   azure         v1.15.1

Here's an image showcasing the issue:

Environment:

• cluster-api-provider-azure version: 1.17.1
• Kubernetes version: (use kubectl version): 1.30.x
• OS (e.g. from /etc/os-release): Ubuntu 22.04