Microsoft.Insights/diagnosticSettings not deployed against existing App GW resource

[VaclavK]

Bicep version
0.19.5

Describe the bug
When instrumenting for Microsoft.Insights/diagnosticSettings with the scope of existing app gw resource, no diagnostic settings get deployed

To Reproduce

var appGWDeployMode  = 'existing'

resource applicationGateWayExisting 'Microsoft.Network/applicationGateways@2023-02-01' existing = if (appGWDeployMode == 'existing') {
  name: appGWName
}
resource appGWNewDiagnosticExistingSettings 'Microsoft.Insights/diagnosticSettings@2021-05-01-preview'= if (appGWDeployMode == 'existing')  {
  name: '${logAnalyticsWorkspaceName}-DiagnosticSetting'
  scope: applicationGateWayExisting 
  properties: {
    workspaceId: logAnalyticsWorkspaceId
    logs: [
      {
        categoryGroup: 'allLogs'
        enabled: true
      }
    ]
    metrics: [
      {
        category: 'AllMetrics'
        enabled: true
      }
    ]
  }
}

Additional context
this works as expected if targeting non -existing bicep resource
we have to use the existing resource construct due to #2316
we have pre-existing App GW instances and wanted to add diagnostic settings hence this existing resource construct

[anthony-c-martin]

@VaclavK please could you open a support case for this?