We’ve seen this behaviour aswell with the Office365 Click-to-Run Outlook version due to the Roaming Signature feature that was introduced.
You can disable this within the registry of the current user.
Hive: HKEY_CURRENT_USER
Key Path: Software\Microsoft\Office\16.0\Outlook\Setup\
Value Name: DisableRoamingSignaturesTemporaryToggle
Value Type: REG_DWORD
Value Data:1
