15.4.5
NOTE: a Player can bump an opposition Player’s body from side-on but any contact forward of side-on will be deemed to be front-on;
They've been paying front-on frees for at least 3 years. It's 100% a free
Hang on, that’s not right. Even if that contact is considered “front-on” (ie. it’s defining an interpretation), the rule prohibiting “front-on” contact says:
(M) bumps or makes forceful contact to an opposition Player from front-on
when that Player has their head down over the football
So in a marking contest, front-on contact is not prohibited.