This is a limitation of the approach used to compute the output passivity index. We use a frequency-domain test which is only valid when "H" has no zero on the imaginary axis or at infinity, see Theorem 11 in
https://www.researchgate.net/publication/347232443_Sector_bounds_in_stability_analysis_and_control_design
Both conditions are not satisfied here, which is why "getPassiveIndex" returns rho = -Inf. You can recover the expected result by adding a small feedthrough to work around these technical issues:
>> [rho,freq] = getPassiveIndex(H+1e-4,'output')
rho =
2.0000e-01
freq =
1
In general, systems where passivity fails to hold strictly tend to be numerically problematic.
