Does a diode block current but not voltage?
Figure 3 from the 1N4148 datasheet.


Diodes have very small leakage current. At 3 V this will be between 3.5 and 10 nA. 
The 2N7000 has a gate−body leakage current, Forward of -10 nA max.
The diode also has about 4 pF capacitance. When the supply voltage jumps from zero to +3 V on power-up the diode capacitance will cause the gate of M4 to jump up too.