Sallen Key Lowpass Filter stops rolling off in simulation
This is a &quot;well-known&quot; issue with the Sallen-Key configuration.
The problem is that the output impedance of the amplifier rises with frequency and starts to look inductive so it's no longer 'fighting' the input signal more as the frequency increases. The output then increases by +20dB/decade until it levels out at the GBW of the amplifier.
You can mitigate this to some degree by increasing the resistor values (and proportionally decreasing the capacitor values), by using a more appropriate op-amp, by using a passive filter in series, or by avoiding Sallen-Key.