Yes, I've noticed similar behavior against regular grenades. But, I always attributed it to the very limited APS range and I guess at least in their case they just flown/dropped slightly outside the APS range. May it be also the case in your situation? (For example, the projectile trajectory was too high, above the APS)
I did some more testing and these things really need their detection radius buffed so that they do not shoot down a rocket after it already hit the tank it is meant to protect.
The only time I use is defending inside objectives on breakthrough, watched a vid where the person was getting loads but mine only seem to get a few, also if I am playing as him in HZ I'll drop one outside extract sometimes it stops nades/rockets getting in back sometimes it doesn't.
