To check that out, I created a test mbm doc and sent a simple HTML message to several of my own addresses, bcc-d to my Gmail account. That worked fine.
So, I went back to the original message (not simple) and sent it to the same addressees. The bcc that arrived at Gmail arrived in plain text. The HTML for the more complex message came from a Google Doc, exported as HTML. That exported html document is a page full of complex CSS and HTML.
I conclude that Gmail gave up trying to parse that complexity and reverted to plain text. So, not a MaxProg problem.
Thank you.