In my experience, the issue you are having with the cropping tool occurs due for one/ both of the following reasons:
1. Under the 'Lens' tab you have a lens profile selected and the 'Hide Distorted Areas' check-box is not ticked
2. You have straightened your image, either manually or using the straighten tool
This 'issue' arises no matter whether you are cropping unconstrained or with a specified ratio.
I also find it frustrating that the crop tool in C1 Pro works in this way, particularly if I have a lot of images to straighten and crop - in fact, in this situation, I have taken to cropping first (so that I can draw on the crop) and then straightening after, but this means that I have to guess how much of the crop area will be affected when I carry out the straightening. At least with Lightroom, when you have straightened an image, you still have the option to select the Crop Tool and draw on a crop, rather than bringing in from the edges.
In fact, I raised this very issue with the guys at Phase when I was using C1 Pro 6, but nothing happened. You never know though, the long-term problem of the 'sticking' crop area was finally fixed in C1 7 Pro, so it might be worth you raising a support case with Phase (I have always found their support team very helpful).
Anyway, this probably doesn't help you much, as the issue is simply a function of the way that the software works but, hopefully, it gives you an explanation for it.