Sallen Key Cut-off Frequency
A sallen key filter like this (or any 2nd order low pass filter) has gain-magnitude of "Q" at the cut-off frequency. Given that your values produce a Q of 0.5 then the amplitude at the cut-off frequency is 6 dB down.

If you instead made the feedback capacitor \$\sqrt2\$ higher at 11.3 nF and the grounding capacitor lower by the same amount (5.66 nF) then Q would be about 0.707 and your filter would have a cut-off frequency of about 1990 Hz and an amplitude of -3 dB down at this frequency.

This would be the ideal (so-called) butterworth response.



You can try it for yourself at THIS website.

Proof that Q is the value of the T.F. at the natural cut-off frequency: -