MOSFET switching with 5V but latching ON with 12V
The CEP50P03 is a PMOS which requires a negative gate source voltage.
If the threshold voltage is (typically) -2V, you should not drive it with -2V, but with a bigger negative voltage.


A fast and easy way to get a rough indication what \$V_{GS}\$ to use: check the conditions that are given for \$R_{DS(on)}\$. In this case: \$V_{GS} = -4.5V\$ and \$V_{GS} = -10V\$.