Decimation with IIR filter altering amplitude of signal
First, 0.3dB is generally considered small potatoes in signal processing.

Second, the documentation for scipy.signal.decimate states that it uses an 8th-order Chebychev filter by default (without specifying the ripple, dangit!).  Chebychev filters have some ripple in the passband; if the bulk of your signal's energy is falling into the troughs of the ripple, then you should expect some attenuation.  In particular, it appears that your filter has a lot of DC content, and even-order Chebychev filters tend to have a gain of less than unity at DC.

So I suspect that it's doing exactly as expected.