I found out you have to use the AES-256-C option for Net-SNMP see here https://github.com/net-snmp/net-snmp/blob/cbf71bf391e88b387fcd0a8875b97433180db7a2/snmplib/snmpusm.c#L196
but this never shows up in the command line listed protocols. I have a small set of patches for my local install that I have been using for a bit, I can look into doing a pull request.