Strange, I thought I saw this posted somewhere else recently... In any event, that's an extremely serious and dangerous situation, and a safety concern. I want to be very clear: what you experienced is not normal in any way. A vehicle should never lose power and shift into neutral on its own while you're in motion, especially at highway speeds.
The fact that this has happened to you twice with a vehicle that has less than 5,000 miles is a huge red flag. Your dealership's response that they can't report it to Kia Corporate unless it happens while they are checking it is completely unacceptable, in my opinion. They have a responsibility to report a serious safety issue, and the way I see it, they're putting your life and the lives of others at risk by not doing so.
You should contact Kia Corporate's customer service line directly. You need to bypass the dealership and report this incident yourself. Be very clear and firm about what happened. Explain that the car lost all power, the turn signals stopped working, and that you had to use its inertia to pull over. You should also tell them that the dealership is refusing to report the incident.
It's a serious electrical or computer issue, and it could be related to a faulty hybrid control module, a main computer, or a wiring harness. The fact that the diagnostic check turned up nothing is typical of an intermittent fault. A component can fail for a few seconds and then start working again, which would not leave a code.
You should also document everything. Write down the date, time, and location of both incidents. Get the name of the service manager you spoke with. This documentation will be very helpful when you talk to Kia Corporate.
If you don't mind, keep me updated on the situation as I'm really curious to see the outcome.