Get-EXOMailbox -Filter returns empty when $WarningPreference = 'SilentlyContinue'

Get-EXOMailbox -Filter returns empty when $WarningPreference = 'SilentlyContinue' under UAMI app-only auth

Module version: ExchangeOnlineManagement 3.x (PS7, Azure Automation sandbox)

Auth method: Connect-ExchangeOnline -ManagedIdentity -ManagedIdentityAccountId <UAMI ClientId>

Reproduction steps:

$WarningPreference = 'SilentlyContinue'

Connect-ExchangeOnline -ManagedIdentity `

-ManagedIdentityAccountId "<UAMI-ClientId>" `

-Organization "yourtenant.onmicrosoft.com" `

-ShowBanner:$false

# Returns empty — mailbox provably exists

Get-EXOMailbox -Filter "ExternalDirectoryObjectId -eq '<EntraOID>'"

# Restore and retry — returns correctly

$WarningPreference = 'Continue'

Get-EXOMailbox -Filter "ExternalDirectoryObjectId -eq '<EntraOID>'"

Expected: Mailbox object returned regardless of $WarningPreference

Actual: Returns $null when $WarningPreference = 'SilentlyContinue'

Notes:

• Only affects -Filter parameter under app-only/UAMI auth
• -Identity and -UserPrincipalName are not affected
• Delegated auth sessions are not affected
• $WarningPreference = 'SilentlyContinue' is a common pattern in Azure Automation runbooks, making this a significant undocumented breaking behaviour for automation scenarios