When the column to be masked is having all spaces, then the FIXEDUNIQUE function is not working and not appending the sequence number.
FDM 4.9.1+
This is a configuration issue where another option needs to be added for columns having only spaces.
Use the below option in your options file:
TRIMVALUES=N
If TRIMVALUES is set to Y (or if it's missing) then all blanks values (values containing only spaces) are retained as they are.