There are two constraints files in the design. The "top.ucf" file is set by default for New Constraints, and the other one contains LOCs for a bunch of pins.
If you add other attributes to this IOB (e.g., Drive, Slew, Termination, etc.), they are written to the "top.ucf", not added to the existing constraints, and the Constraint File cell is updated to multiple.
The tool does not add IOB attributes to the UCF containing the existing constraints. Why?
This is a known issue with Floorplan Editor.