iPhone 6s has a defective battery which caused it to crash at 30-60% battery life. Usually at the most inopportune moments.
That front screen is tough to remove, It's tough! I eventually figured out that I can wedge an Exacto blade into the lower left corner to pry open enough space to jam the spludger in there. That took about 45 minutes. The rest was easy. Except for the glue strips, they snapped right away. I had to bend up the battery just enough to get a better grip on the strips to pull them successfully. With the battery replaced and everything put back together, my phone was working fine for most of the day. I was attempting to drain the new battery from 84% to 10% for calibration by recording a timelapse video for a few hours. When I checked on it later that day the battery was still at 84%. "Battery life on this is incredible." I decided to let it drain through normal use. The next day I noticed there was some lag while typing, apps would close slower, generally it was sluggish. Then the screen turned white and it was stuck in a boot loop. I tried charging it for a couple hours but it didn't help. I then took the phone apart unplugged the battery and plugged it back in, put all the pieces back together and that resolved the issue.
If the phone goes bonkers after the procedure, take the phone apart and try unplugging and replugging everything you touched before.