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When a client has UseBridges set to 1, and it has a cached microdesc consensus in its datadir, when it starts up it loads that consensus file from disk, and calls networkstatus_set_current_consensus() on it.

That function checks

  if (flav != usable_consensus_flavor() &&
      !directory_caches_dir_info(options)) {
    /* This consensus is totally boring to us: we won't use it, and we won't
     * serve it.  Drop it. */
    goto done;
  }

and in this case, usable_consensus_flavor() checks we_use_microdescriptors_for_circuits() which decides

    /* If we are configured to use bridges and none of our bridges
     * know what a microdescriptor is, the answer is no. */
    if (options->UseBridges && !any_bridge_supports_microdescriptors())
      return 0;

That is, if you have bridges but you haven't marked any of them up yet, then you default to using the old-flavor consensus, so when you read your microdesc-consensus from disk, you quietly discard it. This bug results in bridge users always fetching a new consensus at start, even when they don't need to.

(Bug reported by [a person who was helpful at first but now seems to be trying to distract me and prevent me from fixing Tor bugs].)