After/while fixing T7722 I did run into the Error message:

Not all attachments can be shown.

The hidden attachments are:
longname.withextralongextension

The mail exceeds the maximum size GpgOL can handle on this server.

i.e. attach_to returned errocode 0x80004005.
As the attachment was only 80k I checked for the reason and discovered that the sanitizeFileName T6864 can break the extension as they remove "..".
The filename was truncated and ended with "..." plus ".msg" (sometext.sommorelongtext....msg) extension so all 4 dots got removed and the extension was no longer "msg" but "somemorelongtextmsg" which was ok for creating the temp filename but outlook didn't like it.