I don't see how this is a bug, just because it doesn't work how you want or expect it to work. I would say rather than a bug, it's possibly missing an extra feature, such as a tick box to indicate to fire immediately, then use rate of fire.
The turret is doing exactly as it's told - wait 5 seconds then fire. It will then fire every 5 seconds if appropriate.
If you want it to fire immediately, then wait for 5 seconds then:
Set the rate of fire to 1, then on shoot, set the rate of fire to 5. It then works how you want.
Surely that's why the Event system gives us access to the turret parameters at runtime? We specifically tell it to behave how we want it to.