As far as I know, the actual firmware enables JFFS partition on factory reset for ARM based routers (mainly AC ones) due to the need to store some infiltration there for the AiProtection from Trend Micro.
On MIPS based routers (e.g. N66U) it's disabled and not formatted until you do so...
To be on the safe side, you should enable it and also check format on next reboot and (of course) reboot - before you continue your setup.