I am doing the detective run, and I am at level 4; the requirement for level five is supposed to be "solve 2 cases." However, it is telling me I only need to solve one case (same as previous level), ...
Ideas
The game counts all the cases you've solved, If you've already solved one, then you've met the promotion requirement for level 4. You'll need to solve two cases to get promoted at level 5 but that includes the one you already solved at level 4, so it's only one extra case. This pattern continues all the way to level 9, where you need a total of six cases solved to get promoted to level 10, but you'll already have solved at least five cases to reach level 9, so it's only one more case.
The total case requirement won't affect you much if you started at level 1 and worked your way up. However, it can have a significant impact if you got advanced placement by completing a University degree.