This is a feature of Dxdesiger, not the bug. Because you have set the rule (\n|-|) for this property in Central Libary(check it inproperty definition editor), so Dxdesigenr will don't allow you to edit it in such way . This is the righ way to help user to avoid errors.
If you hope to reset the ref des to none, just click the symbol and click this property in the propertis dialog and RMB -delete instance value.
The R? format is a hangover from the netlist flow and is unnecessary in the EE flow. My advice would be to remove such RefDes info in the symbol definition as a long term solution (just remve the dummy value R?). The prefix will be assigned based on the prefix defined in the Part Data Editor and the number
Just leave a place holder for the Ref Designator with an empty value.
ok, thanks to everyone