Yeah I don't know what's the deal with this is, maybe it got replaced by the stun shot (which doesn't fire 80% of the time)
The stun shot always fires afaik, it just fires at the ground where the tank used to be when the ability was enabled. The stun shot seems have a nigh-invisible trail as well- even when I do manage to stun a tank I never see the round leave the pak, only the tank getting stunned.
the pak stun works well with the ostheer puma / stug stun, since those track targets normally
so you can stunlock the stopped tank. it's probably for the better that the pak doesn't have the tracking stun shot- the Pak is already really good, and one pak being able to stun and kill any medium tank 100% of the time would be crazy OP.