Cascaded RC Bandpass filter loading problem?
What you're forgetting is the -3dB cutoff point for the two filter stages. Recall that
$$
f_c = {1 \over {2 \pi R C}}
$$
Reference 1, Reference 2

For both your first stage and second stage we calculate a cutoff frequency of right around 5kHz (4974 and 5488, if I'm doing my math correctly) which means we'd expect each stage to knock the voltage down by about 3dB (0.707 times its original value), and both stages together to knock down the voltage by 6dB (0.707 * 0.707 = 0.5, approximately). And that's exactly what we're seeing.

My math checks out with yours; the second stage has a much higher impedance than the first, so it makes perfect sense that adding buffers would do nothing to change the situation.