What I noticed is that even after increasing the PHP memory Size, it had no effect. I did see though, that this problems happens only when I send a mail that in the BODY contains Text / Attachment / Signature using Apple mail (I have not tried with others) to send the mail
If I do not ad a signature, no matter how many or what size of attachment (within PHP_memory_limit), they can always be seen in roundcube
Another note. Although you cannot see them in roundcube, once I downoad the mail with Apple Mail, the attachments are there, so is not that is not present, it's a display issue between roundcube and apple mail. will try other mail clients to see if is APPLE MAIL or a general Interpretation error in mails with signatures added at the end.