Low Pass Sallen-Key filter has weird response in bode plot
I'm not familiar with multisim, but from the look of things, you're driving a set of op-amps powered with 0-3.3V with a 1V P-P sine-wave centered around 0V (which means it swings from +0.5V to -0.5V).

As such, your op-amps will be railed for 1/2 of the input cycle, which will make your filter behave very strangely. I suspect the fact that it looks like it's working at all may be an issue with the simulation not being very accurate, or behaving strangely when used outside of the op-amp's "proper" bounds of operation.

You need to add some DC bias to your input, so it idles at \$ \frac{Vpp-Vss}{2}\$.