Encrypted Email Not Viewable in Sent Items

[JeffL327]

I have a user that is seeing a strange issue that just started appearing in the past 2 weeks.
We have a sensitivity label that is setup to Encrypt Only when applied to an email.
Once the email is available in her sent items folder, it is accessible for a few minutes.
After that, the encrypted email can no longer be opened.
Has anyone else experienced something similar or found a workaround/resolution?

[#2]

How are the mails being encrypted? Microsoft Purview Message Encryption?

Is this only happening for this one user, or is it reproduceable for all users?
If so, looks likes something is wrong with the implementation/configuration of the encryption.
It isn't a shared mailbox, or a delegate on a mailbox?

[#3]

Yes. The email is using Sensitivity Labels that were created in MS Purview and applied from Outlook. When the message is sent, it is then encrypted. What I discovered is that viewing the Sent Items in Web Outlook, the message was viewable. This only appears to be happening in the old Outlook Client. We enabled the new Outlook Client on the computer and obviously that resolved the issue. I would still like to understand if this is by design as Microsoft pushes everyone to use the new Outlook client or is there a configuration in the old Outlook client that can fix this.

[#4]

2 questions:
- What outlook version are you on? Please provide a screenshot (File --> Office Account). Blur out your user information + connected services + Subscription Product for %account%
- Please verify if the affected account is a shared mailbox or a user mailbox