Problem with switch-on button circuit
First of all, you will need to understand what's happen. You should measure the following voltages:


Q2 gate voltage
Q1 and Q3 gates voltage


Because you used pretty high resistance values for your pull-up and down (R9, R10 and R11), you could have non negligible voltages if you have some current leakage, even small ones.

So, when your system is supposed to be OFF:


If Q1 and Q3 gate voltages are close enough to 0V, you could end up with these transistors to start conducting, which would cause Q2 to conduct as well, and V_in to be partially powered;
If the current leak at Q1 and Q3 is too important, even when they are driven open properly (see the datasheet for maximum leakage current), the voltage drop at R10 could be non negligible, and partially turning Q2 ON. Keep in mind that a 10µA current through a 100k resistance will generate a drop of 1V.


Last solution, if everything should (in theory) work: make sure your board is clean enough.