Multiple MOSFET based AC switching circuits on the same supply
When the circuit is off, q3 and q4 are off.  When you turn on P1, the sources of q3 and q4 are riding on the 220V power line, and the gate is (power line + 5) volts.  This means that GND_ISO and +5V_ISO must also follow the power line, taking Q5 and Q6 with them.  Whatever your load is remains referenced to neutral, so Q5 will have high gate-drain voltage in the wrong direction or just act like a rectifier through Q5 body diode if D2 is not present.

You will need separate isolated supplies.