Okay, thank you for the feedback.
I logged in and played around the settings, and found out that the secondary logic combination was causing the problem. I have switched the logic to AND without secondary logic. It will basically give you the same results order, as the index table automatically prioritizes exact keyword matches, so the secondary logic is not neccessary. It seems to have resolved the issue.
I have marked this as a bug, we will look into this more deeply before the upcoming patch.