ajustable resistance and MP2307
R4, will act like you connect Vout directly to FB, and reach the FB limit (0.950V Max), So Duty cycle of PWM (inside MP2307) will be max.

Solution:change VR R4 to 2 resistors to ground and connect middle of two res to FB pin.


R1=26.1k, R2 = 10k (according datasheet)