Why is this LPF design passing frequencies above resonant in such magnitudes?
I think your calculation for the attenuation is wrong. A gain of -7.24 dB correspond to an attenuation of 10^(-7.24/20) = 0.43. That implies that the 1.4 volts sinusoid should appear with an amplitude of 0.6[V], which match with you simulation.

Remember that voltage gain in dB is given by \$G_{V}=20\log(G)\$. It seems you're using the formula for power gain, given by \$G_{P} = 10\log(G)\$.