Message body changed, DKIM fails when using Exchange Online

1. I send a message from an aol.com address to a gmail.com address. Everything works as expected.
2. I send a message from an aol.com address to an address handled by my organization's Exchange Online and EOP that forwards to gmail.com. The message does not pass DKIM and is placed in the spam folder.

The DKIM message from Google is:

dkim=neutral (body hash did not verify) header.i=@;

When I compare the 2 messages, the only difference in the fields used for DKIM (DomainKeys Identified Mail) validation is that the following line is removed from the message body when handled by Exchange Online:

This is a multi-part message in MIME format.

Just wondering if anyone else has seen this, knows why it is happening, or how to prevent it.

Thanks!

EDIT:

The mail-flow is more complicated than what I initially described. External mail first hits EOP (Exchange Online Protection) and is then routed to an on-premise Postfix server. From there it is sent back out through Exchange Online. It is possible that the message is changed by either Exchange Online or Postfix--I'm trying to determine that now.