Certified branches only receive fixes as a result of commercial agreement customers encountering issues, otherwise it remains a frozen branch until a new one is created (Asterisk 13.8-cert1 was just released). If you are a commercial customer please contact support in order to pursue this problem.
If you are not a commercial customer then I’d suggest tryingthe latest Asterisk 13. If it occurs you can follow the guide on the wiki[1] for getting a backtrace when there is a deadlock and file an issue (with configs and details about what the system is doing) on the issue tracker[2].