The flash_nreset signal will be asserted in any of the following cases:
(1) The device with the PFL design is powered up or configured.
(2) The pfl_nreset input signal is asserted.
(3) When Quartus® II programmer is used to program the flash memory, if the PFL has programming mode enabled.
If you want to assert flash_nreset, reset the PFL using pfl_nreset.