When fixing routing and saving the constraints from the GUI, the FIXED_ROUTE property is not saved into the target XDC constraints file.
This worked in previous releases of Vivado, but is broken in 2014.1.
This is a known issue that is resolved in Vivado 2014.2.
As a workaround, the constraints can be manually saved using the write_xdc command.
Note: For nets that terminate on a LUT pin, you should add a LOCK_PINS constraint (to prevent pin swapping by the placer or router). See (Xilinx Answer 59242).
This is currently not implemented with the Fix Routing command or the Assign Manual Routing flow.
This is planned to be implemented in a future release.