I think mbed tls should support fragmented DTLS handshake messages. At least in v2.7.0 there’s comment in source code that they are not supported.
If considering e.g. some commercial MIB browser that supports SNMPv3 TSM (and used openssl to implement TSM (a.k.a. DTLS)), openssl fragments some of the DTLS handshake messages.
Thank you for your comment.
Mbed TLS does support the max_fragment_length extension, for DTLS.
However, as you mentioned in the comment, it is not fully supported. It supports reassembling messages, specifically certificate messages, but does not send fragmented messages.
This is something that we are aware of, and working to fix in the future.
Thank you for your interest in Mbed TLS!