It may be due to different reasons.
Plese check below ones , which are applicable to your design.
1)DC checks the variable called port_complement_naming_style variable
and accordingly assigns the port name when you are writing the netlist. check the setting and you can over write it. To get rid of this ,you need to use below variable. plese see the man page for required settings. compile_preserve_subdesign_interface
2) use change_names command before writing out the netlist with proper settings for ports.
Immediate solution : If u cant find the cause and need to write it the way you want ,follow below procedure.
a) Run the report_names command with appropriate options , and direct to file called ORG_NAME.TXT
b) Delete all the lines, which you dont want to change as DC produced . Now you will be having only Port names . In the last column called New Name, you add your required name and save it.
c) Run the below command on current design.
change_names -verbose -names_file ORG_NAME.TXT
Even though its lenghty process, it will fix your problem once for all...
Regards,
Sam