I often use Upright and the manual transform settings for perspective correction in ACR and it often happens that the crop rectangle changes its ratio in the progress, either it changes while using the transformation sliders or while moving one edge of the crop rectangle near the image boundary.
Right now I have a image open with manual transformation (a bit of vertical and horizontal) and a ratio of 2:3 and constrain to image on - if I pick the edge ot the crop rectangle and try to push it too far over outside the image boundary I can push the crop rectangle to something like square crop.
Obviously I would like the crop ratio to not change at all - now I either have to be very careful while cropping and using transform or I have to check the finished files for any slight deviation.
This problem persists for a few ACR versions now. Complete reinstall while switching from Windows 7 to 8.1 with no change. I have a three monitor setup, I dont know if that has anything to do with it.