Hi
From my understanding we can analyse the curve f vs vin.
We know f(frequency) = 1/2*pi*Rout*Cm
Note: Rout is output impidence looking at either Vout+ or Vout-
Cm is Miller capacitance at Vout+ or Vout-
Rout = 1/gm(pmos)
Cm = (1 + Av)Cgd(nmos) with Av is gain of op amp = gm(nmos)* Rout = gm(nmos)* 1/gm(pmos)
gm(nmos) = W/L*Un*Cox(VGSn - Vtn) = W/L*Un*Cox(Vin+ - Vx - Vtn) Vx is voltage at drain of nmos with Vgate = Vbn
Now we see if Vin+ increase ---> gm(nmos) increase ---> Cm increase ---> frequency decrease
That why your sim is correct
Added after 4 hours 10 minutes:
If you increase Vbn or Vbp the result the same because current bias increase make gm(nmos) of input op amp increase