There must be something wrong with either your computer's networking options, the sequence of command you used, registration to the server (wrong key, or a device with the same identity is registered with a wrong key. You can check this out at https://leshan.eclipseprojects.io/#/security) or something else.
It should not. But you maybe found a bug ?
Thursday I integrated in master (and so applied on Leshan
sandbox) some commits about integration of the new Californium
version, there is maybe a regression with this. (Californium
itself or my modifications)
This could help if you provide more context. (which client used
? DTLS or not ?, if DTLS, rpk psk or x509 ? providing wireshark
capture or log could help too)
If you prefer you can use github issue ? (Mailing list is fine
too)