I had this problem in the previous Version of Lightroom too. Was hoping it would go away when I updated. I am using a Mac book Pro with 10.11.6. It just started happening mysteriously a couple of months ago. Whether I crop the image or not,when I export it from LR the image is cropped more than what I specified.
More details and a screenshot of your export dialog would be helpful.
As I was preparing some screenshots I noticed that an Instagram export plug-in had added a forced crop step to the export dialog. I deleted that and the problem went away. Thanks.