My 2018 Forester has been doing that since I've owned it. Doesn't matter if I press the hatch button long or short duration; I end up staring at it then go to the key remote.I ran into the same issue, and fixed it the same way: hold in the button on the liftgate itself for at least 7-8 seconds. The gate will eventually pop open, and you can lift it yourself.
If the problem returns, I would strongly encourage you to take it back to the dealer and request a warranty repair. Even though this rates "mildly annoying" for most of us, and even though there might not be an immediate fix, it will help all of us to start a paper trail with Subaru.