Hmmm, just encountered what seems like a serious bug. I couldn't see a friends level when searching in the "Made by Friends" tab, so I popped out to the PS4 menu, closed the application, and booted it back up. Upon booting up, a message popped up saying "Your save file is corrupt, press triangle to delete save and continue". Obviously I didn't want to do that so I restarted the game. Same message popped up. Reluctantly I pressed triangle, hoping the game wouldn't actually do what it said it was going to do, but it did. Upon reaching the menu, my save file was gone, and all my progress with it. Thankfully I hadn't gotten too far into the N++ levels, but I had completed all the tutorial levels, and all those scores are gone.
I'm not the most anal retentive gamer, so I can just pop back to where I was and continue on, but I'm pretty annoyed regardless. Never seen something like that happen before.