PIC: Watchdog timer off by a factor of 4
According to the datasheet you provided, at page 105, the value you should program is: 01110. Thus the whole register should be 0b00011101 -> 0x1D.

It seems that you already did so. But are you sure that the definition of WDTCONbits.WDTPS doesn't contain a bug ? Try to assign to the whole register directly.

The watchdog is powered by the internal low frequency oscillator. It has nothing to do with the PLL.