Same problem here.
Against the valuable contributor Massimo we found it´s a problem on client site.
Same sender, different recipients all working gw 2014 r2 sp3 client: some get a correct mail, displayed as signed and containing attachments.
Other recipients found in mailbox the mail only without the signs fo signing and attachment. However, if they open this mail, an attachment named "smime.p7m" is shown. If this attachment will stored on harddisk named "smime.eml", groupwise can open this file and shows correct content including attachments.
So we are in research, because the groupwise client will not recognize the signing certificate.