PIC32 SPI Flash Burst Write chip select behavior
The behavior you observe is due to SPIxCON.MSSEN bit being set ( it is cleared by default so apparently it was you who set it blindly before understanding how it works - sorry, couldn't resist ). In this mode SS stays asserted as long as there is data to be transmitted, and it will stay low if you happen to write to the buffer during transmission. This as well as other useful and fascinating things can be learned by studying Section 23 of PIC32 Family Reference Manual, a link to which is conveniently provided on any PIC32 micro product page at Microchip web site. 

You don't have to do this though. Just leave this bit in its default state and drive SS manually.