Sallen-Key topology LPF (Butterworth)
Up to about 100kHz, it all goes as expected.

Unfortunately, after that, the op-amp output impedance rises so much that it's unable to control the feed-forward path that comes through R1 and C2.

Consider increasing the impedance of the filter components, so use 8.2k and 1nF, which should push trouble up another decade in frequency. A faster op-amp would also help. Also, try an 'ideal' op-amp in simulation to see the problem go away.

Another one to consider is the 3rd order Sallen Key, which has an RC input, which puts a true passive 6dB/octave on the response, which may disguise the bad thing sufficiently to be acceptable.