When trying to setup and use the "Endpoint: MIP Classification" automated response rule with the DLP Endpoint Agent and the "new" Outlook UI, the response rule does not trigger and no classification label is recommended or applied automatically. However, you may get an incident generated.
DLP 16.1+
This is working as designed as support for the new "Endpoint: MIP Classification" response rule has not been added to the new Outlook UI mode at this time.
For now, the "Endpoint: MIP Classification" response rule is not supported with the new Outlook UI, however you can use the Block, Notify, User Cancel and Data Retention automated response rules with the new UI. The "Endpoint: MIP Classification" response rule continues to work in the "Classic" Outlook UI mode.