With both Microsoft Outlook for Office 365 MSO (16.0.11929.20144 + 20234) 64 Bit
If I view the email using Windows 10 Mail (Exchange Online) I see the signature.asc attachment. In Outlook is is sometimes hidden by GpgOL. In the inbox or with the email opened GpgOL does not show anything about a signature - the email looks as any other unsigned mail. Sometimes during the past months it even empties the mail - so I have data loss. Not that good… I attach one of two todays signed emails copied to the desktop by drag & drop + zipped.
[Update:] That's funny: only the 3rd newsletter today was processed correctly - see screenshot. And I attached it as .msg, too.