I have just updated from Altium 17 to Altium 18 and I am seeing a strange behavior where the special string text overrides the the actual size of the text.
This special string text size is being interpreted as part of the entire component, and therefore I am getting DRC errors.
I have attached in image that shows the the designator (U29) is completely inside the part, but when the part is selected, Altium interprets the size of the component with the special string ('.Designator') and is therefore much larger than it needs to be.
I had this issue in version 17 as well, but I don't remember how to change that setting.