Infinite scroll issues on search results

Activating the Infinite Scroll plugin in the 2014 theme (no modifications) is giving strange results for the search results page.

10 posts per page (under reading settings)

• Infinite scroll disabled: 18 post results for search term, split over 2 pages
• Infinite scroll enabled: 20 posts results, the first two in the first set of loaded results are duplicates from the first “page” of results

5 posts per page (under reading settings)

• Infinite scroll disabled: 18 post results for search term, split over 4 pages
• Infinite scroll enabled: 13 post results, no duplicates but missing 5 posts

Any idea what might be going on here?

cheers,
Matt

https://www.ads-software.com/plugins/jetpack/