Poles of system seem counter intuitive

I have a system with the three poles 0+8i, 0-8i, and -0.5. When I input a sinusoidal signal, this is the output:Based on the poles, -0.5 is in the left side of the plane so it should be stable. +8i and -8i are on the axis so the output will oscillate. I am confused on why the signal is gaining magnitude as time increases? Based on the poles I would not think this would be the behavior. I thought it would be a steady sinusoid. Where is the amplification coming from?