As far as my understanding goes, the lesser the SCL rating, the GREENER the email is. This rating what you say is opposite if this part of the header stripped the attachment, it should have done the other email.
I assume, you got the headers for the 2 emails from the recipient and I would ask the recipient to check with their admin at their archiving\filtering solution that they have implemented.
As an admin for O365, you can check the FOPE message tracking but that is going to show only whether the message left or not but NOT with/without attachment. So there is not much that you can do.
It is mostly the other end stripping it. Inside the archiving software, there would be options to check for Content Type: ASCII7 /8 UTF and they should check those settings and find out the reason for MAPI based submission emails with attachment stripped.