2nd order Sallen-Key low pass filter frequency response
At above 100 kHz you can begin to write off the op-amp's output as playing much of a role in producing the output signal. This is because it runs out of open-loop gain and, in effect becomes just a passive resistor load at the output node: -

At 1 MHz the open-loop gain is unity and the op-amp just doesn't deliver the goods any more but it's still having a go up to about 200 kHz but it's fighting against the 1 nF capacitor (C1) that connects to the input via R1.
So, as frequency gets higher, C1's impedance gets lower and the OP777's output impedance becomes, in effect, just a resistor to ground with no significant signal to contribute. At this point, any signal seen at the output is due to C1 putting it there via the input resistor R1. The dip in the attenuation at around 200 kHz is just a nuance of one thing conspiring against another thing. For instance if you chose the frequency setting capacitors to be ten times lower and the frequency setting resistors ten times higher then you'd see a better attenuation characteristic from 200 kHz but you'll still get a plateau as the op-amp runs out of steam (maybe 100 kHz higher). If you don't believe me, try it.