I had this problem with my Samsung J2. It seemed to be randomly shutting down during the night, causing me to be late for work on many occasions. I tried all the steps suggested by all the “experts,” checking the battery was installed correctly, doing a restart ( which made no sense considering that's what the phone was doing anyway ), checking apps that I had installed…nothing worked. Therefore, I started tracking when it did this and realized that it was happening every Wednesday morning at 3 AM, so I knew it had to be a setting on the phone. Honestly, by accident I found “auto restart” hidden in the settings, and it was set for 3 AM, every Wednesday. What was happening with my phone though is that I had a security code to open my phone and it couldn't restart after it shut down. Sadly, I’m pretty certain I set it to do this after reading that phones need to be rebooted occasionally for optimum functionality, but for the security code it wouldn't work properly. I hope you find this helpful.
Go to Settings, scroll to Device Care, touch the 3 ellipses top right corner, choose (duh) Auto Restart, and choose OFF.