In short, without setting the timeout parameter, calls to udp_with_fallback will wait forever if, e.g., our initial resolution request or response packet gets particularly unlucky and is lost.
More discussion on how this can become a practical concern in T393600#11207352.
As noted there, other resolution calls in Discovery that use the cached per-nameserver resolver.Resolver instances do not have this problem, as they get the poorly documented default timeout of 2 seconds (see BaseResolver.reset).
As noted by @Volans, it's probably reasonable to set a sensible default at the callsite. Perhaps 2 seconds, by analogy to what the cached Resolvers do.