I suspect there is an error with your Mamiya's onboard meter for their to be blown highlights on reversal film and nothing untoward visible on negative film. As the second response states, latitude of negative film will let you get away with murder, while reversal film must be nailed within 0.3 to 0.6 stop within target light.

Investing in a spot/incident/reflected meter will gear you up for more advanced camera-independent analysis of a scene which is critical for reversal film. Spot metering on negative film will show up deficiencies in metering technique, but incident metered scenes will not because of the grace of latitude.

Once you have a separate meter e.g. one of the Sekonics, run tests using both formal methodology for metering (high, low and basal midtone areas then average or shift). Transfer that setting to the camera on E6 film and note conditions for later reference. Repeat the same scene using the camera's meter. Used skillfully, a handheld meter will outperform an in-camera meter. Things can go wrong with meters e.g. you might forget that baseline calibration has been made on the meter (this is very common in both studio and landscape work e.g. a +1.4 or +2.0 stop to either account for filter use or out-of-range ISO, and end up with a grossly blown or under-exposed scene. Eckó! Great tools to use, but always walk-through before an important shoot!