When you partition the entire FlexNVM space as EEPROM backup, it means that there is no D-FLASH, only E-FLASH(EEPROM backup space).
The E-Flash can only be accessed through FlexRAM(EERAM).
if part of FlexNVM is partitioned as D-Flash and the other part is partitioned as EEPROM backup(E-Flash),then D-FLASH can be accessed normally by the debugger.