We've been towing a 2013 Ford Fusion Hybrid four down for 4 years now with no problems, and never any accrued mileage when towing. The Fusion Hybrids are very, very similar to the CMAX, they use pretty much the exact same (hybrid) drive train.
I think the 'key' to it is (pun intended), like already said, you must tow with ignition in ACC position, not ON. If its in ON position, yes, you will accrue mileage (I know 'cause I accidentally did that once, haha).
If you have the keyless ignition/push button start, its a whole different ballgame, and the procedure to get it in the right mode to tow is different, and that may be where something is not being done right, causing miles to accrue. Bottom line, though, is, that should not be happening.
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")