Hi,
Thank you very much for the detailed description. I have tried to replicate the issue on our ends with similar configuration, but no luck, everything works as supposed to. I will have to debug this directly, as the exact same setup (post types, posts, number of posts etc..) we can not replicate as-is. The issue can be a conflict of some sort, or an unknown bug as well.
Can you please add temporary FTP and back-end details, so I can both change the plugin files by adding debug queries, as well as change the search settings temporarily if needed?
Thank you!