This is a known issue when applying pitch randomization to a Single Sound. As you discovered, a higher pitch means the sample plays quicker, however the trigger region’s length does not change. Single Sounds always loop by default (in order to allow extending the trigger region rightwards), and as a result you may hear the sample begin again before the timeline cursor leaves the trigger region.
Unfortunately there is no way to change the default looping behavior of Single Sounds in the current version of Studio. The quickest fix is to avoid Single Sounds when randomizing the pitch of a one-shot sample, and instead use a Multi Sound.
Just right click on the timeline and select “Add Multi Sound”. Then drop your one-shot sample in to the Multi Sound’s playlist. You can randomize the pitch of the Multi Sound, and manually control its looping behavior (off by default). You should find that the sample no longer repeats if it finishes early.