Potential bug using stdist with an ellipsoid?

By definition, the standard distance should be 0 with only one set of lat/lon coordinates. This is true when I use referenceSphere('Earth') as the ellipsoid, but I get a nonzero answer when I use wgs84Ellipsoid instead. Is this the expected behavior, or is there a bug in stdist when it comes to ellipsoids? If this is the expected behavior, what is the explanation for returning a nonzero standard distance when there is only one set of coordinates?

Edit: Using Matlab R2017a

>> stdist(42.293684,-83.237701,referenceSphere('Earth'))

ans =

0

>> stdist(42.293684,-83.237701,wgs84Ellipsoid)

ans =

2.1261e+04