Yes, the exact same issue occurred with my Kona EV. I was at the grocery store on a hot summer day and I loaded my trunk with groceries. When I went into the cabin to drive, I received a warning light that the trunk was still ajar. I went out and closed it harder...went back inside...same message. Hmm...I drove off thinking it was a sensor issue and my instrument cluster was blank...completely blank... Like yourself, I used the heads-up display to see what my driving speed was. I went home, turned off my car, unloaded the groceries, and when I went back to my car and turned it on, everything was fine! I went to my local Hyundai dealer yesterday to report the issue. They suggested I keep track of it and to let them know if it should happen again. I'm glad (sorta) that another Kona EV driver is reporting the same issue. I always think it's just me.
Another issue I've had is when plugging in my iPhone X using a cable and then placing it on the self-charging pad, the entire display (8") goes blank and my iPhone does as well. This has happened to me twice. I also notice that the phone tends to get very hot, causing the phone to have very odd behaviors until it cools down. Hmm...