I'm running Outlook for Mac build 16.66 and the problem isn't fixed. Something has changed though:
I'm sending and e-mail that is digitally signed from account A to account B within the same new Outlook environment. When the e-mail is sent with an attachment the e-mail received at account B has an empty attachment named "smime.p7m" that is 0 kB. If I stop Outlook and reopen it, it shows the correct attachment for a couple of seconds then it changes to "smime.p7m".
UPDATE: I've discovered this behaviour is only shown in the preview pane of the new Outlook. When I doubleclick to open a message I do get a perfect attachment in the new window. Closing the message the attachment is perfectly shown in the preview pane until I go to another message and back. After this, the attachment is failing again.