Simple resistance based water sensor works in stagnant water but blows out in running water, under faucet
Your transistor gate is being blown by static electricity generated in the flowing water stream. The 4069 does not blow out because it has protection circuity on its inputs while the gate of your transistor is a little "exposed". You can add protection circuitry as follows. Series resistor from input to mosfet gate bigger is safer but will reduce sensitivity you need to decide the balance here start around 1k. One diode cathode to supply+ anode to gate. One diode cathode to gate anode to supply-. These are small diodes they will carry very little power and are normally reverse biased.