az deployment group what-if results in ValidationError: InternalServerError

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az deployment group what-if`

**Errors:**
```
ValidationError: InternalServerError - Encountered internal server error while processing the deployment what-if request. Diagnostic information: timestamp '20201005T130553Z', scope '/subscriptions/REDACTED/resourceGroups/REDACTED', tracking id '3dc7c4d0-073b-4c90-bd3d-4fbc88e9b90b', request correlation id '4fb2562e-9309-426b-8f97-98195665c1dc'.
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- None, deploying an update to pre-existing resource group with 80 keyvaults.
- `az deployment group what-if --resource-group $resourceGroupName -f $templateFile --exclude-change-types Ignore NoChange`

- sidenote, fails with same error in pwsh `Get-AzResourceGroupDeploymentWhatIfResult -ResourceGroupName $resourceGroupName -TemplateFile $templateFile -ExcludeChangeType 'Ignore', 'NoChange'`

## Expected Behavior
Expected to see result of incoming changes excluding change types specified.
## Environment Summary
```
macOS-10.15.6-x86_64-i386-64bit
Python 3.8.5
Installer: HOMEBREW

azure-cli 2.12.1

```

## Additional Context
Have been deploying this two ARM template files with ~13600 lines for a couple of weeks now with no problems from Azure Pipelines. Started failing 30/9 in production while still working in stage.
Today it started failing in Stage as well.
Templates that we attempt to check with 'what-if' have been validated and deploys successfully.
Normally deploying this from Azure Pipelines, but can reproduce locally from my machine.