If you change an FK on an object and discard this change (Button Discard), the FK is not reset. For "normal" columns the value is reset correctly.
This is a product defect (32387).
The discard apparently works correctly, only the display is not updated! So this is actually only a cosmetic problem.
This will be fixed in a future release of the product. If you require this immediately corrected, please contact support for a hotfix referencing the defect ID 32387.