Interesting. Thanks for reporting this. This happened in the past because images had a "content-id" (so they were marked to be an embedded image) but were not really embedded. I did not have a very good fix then because it is hard for us to detect (easy for Outlook itself though) so there might be more special cases where this happens.

It is like if you embed a small image in your mail you would not wish it to be listed as an attachment.
What is your sending application?
To confirm that you can switch your Outlook temporarily to only show text emails. In that case the attachment should show.

Could you be so very kind as to write yourself a testmail that shows this Problem (without private data) and then enable Debugging in GpgOL Options. Log Level "+outlook api" and check the "Include Data" option. To analyse this I sadly need the include data option.

You can attach the log file here or alternatively you can send me such a mail to "aheinecke@gnupg.org" (so that I can reproduce it myself) or just the log to "aheinecke@gnupg.org"

Prio -> High as this is "kinda" data loss (well data is not lost but only hidden).