At defaults the facet.mincount = 0 while the facet.pivot.mincount = 1.
Within the pivot facet code, a facet is made by calling "getTermCounts" which adheres to the facet.mincount and facet.limit. When sorting by index it is very possible for this call to return a whole bunch of results with a count of 0 before any count of 1 results showed up. The facet.limit can be set so that all the results from the "getTermCounts" call are below the facet.pivot.mincount, resulting in nothing being attached to the pivot facet result, which is clearly incorrect.
In any situation where the facet.mincount is lower than the facet.pivot.mincount the results will be incorrect.