That's strange that the behavior is different when the Person ID is not the main sort.

I have noticed that if you select the custom report to include alternative facts, the occurrence of an alternative fact triggers a second, third, etc, line for a person. One has to select to use the principal fact in two places, the "options box" at the bottom of the facts pick list and also the global "use primary facts" checkbox. (This is how one gets only the primary Name fact, but multiple residence facts in various reports.)