Usually, passive circuits as your BPF, are matched to 50 ohms at their ports.
However, active circuits as LNA's, even the designers want to match their input/output ports to 50 ohms, not always get there.
The LNA in your case may have 50 ohms input impedance (working fine with the BPF at its input), but its output impedance may be different than 50 ohms, and when connect the BPF at the output, results lower overall gain.
BPF filters also, if doesn't see close to 50 ohms at their ports, their insertion loss and ripple performances may be affected.