I have not used Inventor
I was going to say it's a graphic resolution setting that needed tightening up, until you stated that the actual toolpaths also follow the shape.
Just to make sure its, can you follow the toolpath endpoints and linetype ( line or arc type ) when single stepping through the backplot toolpath ?
There can be errors in converting, for example, where one CAD system may allow a 0.010" cusp tolerance when exporting, and the importing CAD system has the same 0.010", so you could have up to 0.020" cusp from the original design.
Please explain the steps you follow to actually have the model in mastercam
and any tolerance settings that can be set at each stage.
In Inventor, can you create solids ?, then "Save as" as a STP file ?
Solids tend to be more stable when taking then into another CAD package, where surfaces on the other hand, seem to be approximated, miss bits, not join together blah, blah, blah
You would get better sufaces and curves for toolpathing if they were created within Mastercam when using the imported solid as the base shape, if needed
IMO, import surfaces only, if the solids have errors.