General Description: Some constraints may be ignored after exporting constraints from an FPGA Express project and subsequently re-importing the constraints. This problem has been seen with setting the default slew rate to 'fast'.
This is due to a bug in the FPGA Express 3.3. If a new project is created and constraints are not imported into the project, any Express constraints added to this project will be exported to the design. This problem is fixed in version 3.4.