Boot Diagnostics Not Switching to Managed Storage Account

### Description

We initially enabled Boot Diagnostics using a custom storage account and were able to access the Windows desktop successfully.

Now, we are attempting to switch Boot Diagnostics to use a managed storage account instead. As part of the process, we followed these steps:

Disabled Boot Diagnostics:
$VM = Get-AzVM -ResourceGroupName $grp_name -Name $vm_name
Set-AzVMBootDiagnostic -VM $VM -Disable
Update-AzVM -VM $VM -ResourceGroupName $grp_name

Re-enabled Boot Diagnostics (expecting it to use a managed storage account):
$VM = Get-AzVM -ResourceGroupName $grp_name -Name $vm_name
Set-AzVMBootDiagnostic -VM $VM -Enable -ResourceGroupName $grp_name
Update-AzVM -VM $VM -ResourceGroupName $grp_name

However, after re-enabling, the VM continues to retain the previous custom storage account setting instead of switching to the default managed storage account as expected.

We would appreciate assistance in resolving this issue and ensuring Boot Diagnostics uses a managed storage account.

### Script or Debug output

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### Environment data

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### Error output

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