Using older versions of Outlook for Mac for personal email and calendar
I'm on 16.66 (22100900), update channel is Current Channel (Preview) and I just tried to send an email that had a digital signature and a PDF attachment to myself, and on the other end I didn't get the attachment, but the digital signature appeared as an attachment on the other end.
If I send the same email, disabling my digital signature, with the same PDF attachment, I get it on the other end.
So, no, 16.66 (22100900) doesn't appear to fix it.