I'm not sure what is going on, I have the exact same issue, and it appeared with the latest 10.13.4 update for me on a 2015 MacBook Pro. I just contacted Apple Support, did the PRAM and controller resets to see if it would still persist and it did 5 minutes later. I pulled an older thread from Mavericks with the same issue that did not seem to provide a solution (a few had suggested Bluetooth pairings, which I deleted all of them) and the issue continues to occur probably every 5 minutes or less, depending on the day. No hot corners, just full screen Safari, messages, apps, or any program that supports full screen. It is really bizarre and sounds like a slight problem that is a huge annoyance. I will type an entire sentence and nothing will show up in messages because, surprise, the full screen imessages program became inactive, but no other program opened up. I have an open ticket with Apple Support and I will update when I have one, but for now it’s process of elimination, deleting programs and seeing who is the culprit. Just curious, do you have Microsoft Office and OneDrive on your machine?