The battery on my 2015 15" MBPr was on its last leg so I took it in to get a new replacement. Whenever I disconnect the power cable and other times just randomly, my entire MBP freezes. Even the Finder. Pretty much every hour. I just leave it hanging and after about a minute it reboots itself with the all familiar messages as seen in my screenshots. One time I had the folder with a question mark so I made sure everything was backed up once I got it into Safe Mode. I reset PRAM etc. I could reinstall the OS but that shouldn't make a difference. Either your OS is working or it's not. Either there is something wrong with the battery/disk or there isn't. I did a repair from the macOS utility but everything was fine. I'm going to bring it in AGAIN so they can look at it, but in the meantime what do you think it is?