Change in gain and cutoff frequency of 2nd order passive filters due to amplifier circuit
The low pass filter has two series resistors of value 1 kohm and when you add-on the impedance presented by the inverting amplifier (5 kohm) you will modify the cut-off frequency of that filter. The gain will also lower because the 5 kohm forms a potential divider with the two series 1 kohm resistors.

A way to avoid it is to make the 5 kohm resistor much higher in value. It connects to a virtual earth summing point so it is effectively acting also as a resistor to hard 0 volts.