Just wanted to let anyone know that may be seeing the same issue and has received user complaints.
We are currently utilizing LogMeIn for our now remote workforce. When users try to view a Do Not Forward email coming from within our tenant, Outlook (desktop version) with Reading Pane on will turn black (Windows 10 v1909 and earlier) or turn invisible (Windows 10 v2004 and later). If the Reading Pane is turned off, the opened email will exhibit those behaviors while Outlook is still visible. I have contacted Microsoft who has replicated the issue and has no known fix. I have also tested and confirmed this in TeamViewer as well.
This does not occur if the user is connected via RDP and appears strictly limited to 3rd party tools. The only current workaround is to view those emails in OWA as the web app does not exhibit the above behavior. Also, regular encrypted emails (in-and-out of tenant) and Do Not Forwards returning from outside the tenant are not affected.