At the moment, Bro only seems to attach UDP analyzers based on signatures, if the very first UDP packet matches the signature. Even if later UDP packets match the signature, the analyzer is not attached.
The attachments contain a test case. f1.pcap contains a DTLS connection with a few STUN packets that are sent first, which is not recognized as DTLS. f2.pcap contains the same connection with the first few packets missing.
It would probably be nice if one could at least opt to attach analyzers at a later time too, if a signature matches. (I know that 2.4 is probably a bit optimistic for this).