OnePlus has been in contact with the original user who reported the 911 reboot bug, and they have successfully tested a bug fix that has stopped the phone from rebooting when dialing emergency services. It is expected that they will release an OTA update in the near future.
https://www.notebookcheck.net/OnePlus-5-OTA-update-for-the-emergency-call-bug-expected-soon.235902.0.html