Hey.
I believe the ADS-C reports show the indicated mach number as cleared rather than the true mach number you might actually be flying. Don’t quote me on that though but I would believe that it’s similar from an ATC perspective as IAS vs. TAS, you fly a certain TAS to comply with a cleared IAS and that changes with altitude etc etc. You’re cleared M.083 and you are a true M.0812 to fly an indicated M.083 as input into the FMS then you are considered and shown as doing a M.083.
ADS-C reports show if there is a discrepancy between actual vs. cleared route for instance. If a different fix/route is inserted into the FMS than is cleared a message promts and advises the controller. Would probably be the same action if a report shows a different mach than cleared.