The only thing that comes to mind is software or hardware. I would back up the phone on smart switch, then run factory data reset. I would test it first before actually restoring the data back on the device. If it seems to be fine, I would continue to restore the data EXCEPT for Apps. Test again and see if everything is working accordingly, then proceed with Apps to see if the issue persists. Check if there are any updates on the software.
If it does persists after factory reset, it could be a chip related issue (assuming the device has not damages on it). If possible I would try to push for Warranty if that still applies and notate all the test you did.
Hopefully this helps. Good Luck