A shirt under bibs is nothing, I see that look semi-regularly.
Once was riding on the MUP where an older dude was riding with bib straps over his oversized long sleeve jersey, partially stuffed down the back side. It didn't occur to him that riding with inaccessible rear jersey pockets would be an invalid configuration. My best guess was that he wanted to show off Assos' silly graphics that they put on their bib straps.