Label Inheritance in outlook.

Hello Manu2360,

There is no documentation that I could find for this exact scenario. Logically this should work, unless,

1. Once the attachment is removed, before or after adding another attachment if the label is touched manually like clicked Sensitivity and selected/confirmed/changed.
2. If the second attachment is not a local physical file.

Also try this in outlook web and confirm if you are experiencing the same behavior. 

Refer to: https://learn.microsoft.com/en-us/purview/sensitivity-labels-office-apps#configure-label-inheritance-from-email-attachments 

Additionally, after reproducing the behavior, you can check the audit logs to check the label activities. 

Refer to: https://learn.microsoft.com/en-us/purview/audit-log-activities#sensitivity-label-activities 

https://learn.microsoft.com/en-us/purview/sensitivity-labels-office-apps#auditing-labeling-activities 

Refer here for known issues with sensitivity labels: Known issues with sensitivity labels in Office - Microsoft Support

Hope this helps!

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