Here's what I've discovered about the rear-hatch button on the remote:
If you maybe have the cargo area full of stuff and you're crowding the back hatch so it can't close fully, or if there's nothing in that area but someone tries to close it very gently, in either case it winds up in this half-latched position. When it's in this position, you can tell by observing that the hatch isn't quite flush with the body of the car, and the dashboard will probably signal to you that the hatch is open if you try to start the car. But when it's half-latched like this (at least in my experience), you can neither close it fully, nor can you use the button on the hatch door to unlock it so you can open it and slam it shut. On the four occasions when I've encountered this half-latched situation, for some reason, the button on the remote made it possible to open the hatch and slam it shut, when the button on the door would not open it.
On a slightly tangential note: a friend of mine has a Toyota Van with a button on his remote to open the rear hatch. Unlike the Niro, his hatch is on a motor: it goes up and it goes down under motor control. And the linkage gets whacked if someone makes a mistake and tries to close it manually. I told my friend that I didn't have a lot of appreciation for that feature (after I had mucked up the linkage on his van by naively trying to close the hatch manually). But he told me that he loves that feature, because he can walk out of the grocery store with a bag of groceries in each arm and so long as he thought to put his remote in his hand before he walked out, he can hit that button and the hatch opens for him. This leaves me wondering about all of the times my Niro has started the alarm horn because I left the key fob in my pocket and when I sat down, something pushed against that button. If I owned my friend's Toyota van, perhaps the back hatch would be opening every time I sit down in my office chair, and then maybe, because it has a motor on that door, after a minute or two of inactivity, perhaps it would automatically close, just as the Niro's door locks will automatically relock after a period of time if you press the unlock button, but you fail to open a door. Think about it.... In other words, it does seem to be a strange feature on the Niro key fob, I can kind of see why it doesn't actually open the rear hatch, and when I think about my friend's situation and also about all the times the car alarm has gone off because I've got too much stuff in my pockets, I'm kind of glad that it behaves the way that it does. But better still would have been for Kia to provide a key fob where the buttons were somewhat recessed, and to provide a latching mechanism for the back hatch that would open the hatch from a hafl-latched state via the button on the door.
2018 Niro PHEV Gravity Blue