You are right, I think I found the problem.
By default, the plugin stops the search process whenever finds the sufficient amount of results to conserve performance. There seems to be an issue with this when multiple sources are selected, specifically the attachment and post type sources at the same time – I can replicate this on our test servers as well.
The quickest and most effective solution for now was, that I increased the number of results a bit, to 40 at a time, that should increase the potential results pool, and improve the matches a lot: https://i.imgur.com/tvEqmtE.png
It is an effective bypass to the issue for now.
I will make sure to resolve this in the upcoming release completely – it should be out within a week.