Orders of complimentary butterworth filters for IMU
You are simply seeing the quantization of the double type. Basically you have a pole and a zero that should cancel each other out. But because of the limited precision of the double data type, it doesn't. So instead of having 0 dB you have a gain of 10^-15 dB give or take.

But in real life,  99.99999% of the time 10^-15 dB = 0 dB

For the record, I think you could use the minreal function in Matlab to cancel the almost equal pole(s) and zero(s). You would then get your unity gain and your flat bode plot with 0 dB and 0 degree phase.

I don't like simply posting links, but a reminder on how floating-point works should help you.

https://en.wikipedia.org/wiki/Floating-point_arithmetic