The SNMP engine ID may be incorrectly set to 00 00 00 00.

The Simple Network Management Protocol (SNMP) has the following configuration, but the last four digits, which indicate the local engine ID, are all zeroes.

set snmp engine-id use-default-ip-address
 
@router-re0> show snmp v3
Dec 01 21:17:52
 
Local engine ID: ff 50 12 65 c8 00 00 00 00      <<< The last four digits are zeroes.
Engine boots:           6
Engine time:     77573082 seconds
Max msg size:       65507 bytes
 
Engine ID: local
    User                            Auth/Priv   Storage      Status
    IV                               md5/des    nonvolatile  active       
    NSATMS                           md5/des    nonvolatile  active       
    Smarts                           md5/des    nonvolatile  active       
    jspace                           md5/des    nonvolatile  active       
    Arbor_CP                         md5/des    nonvolatile  active       
 
Group name           Security  Security              Storage      Status
                     model     name                  type
jnpr-router          usm       Arbor_CP              nonvolatile  active       
jnpr-router          usm       IV                    nonvolatile  active       
jnpr-router          usm       NSATMS                nonvolatile  active       
jnpr-router          usm       Smarts                nonvolatile  active       
jnpr-router          usm       jspace                nonvolatile  active       
jnpr-router          usm       nms-common            nonvolatile  active       
 
Access control:
Group                Context Security      Read       Write     Notify
                     prefix  model/level   view       view      view
jnpr-router                   usm/authent  router-vie
jnpr-router                   usm/privacy  router-vie

The default IP address is null during SNMP Engine ID generation.
And it is a very corner case and a timing issue which might be seen after Route-Engine replacement activity.
To restore the engine ID, perform the following steps by deleting the engine ID configuration, then add it again:

set snmp engine-id use-mac-address 
commit

set snmp engine-id use-default-ip-address
commit

Note: Re-configure all settings under SNMP V3 USM level.