proxy_license now raises ClickException instead of RemoteError (a SystemExit), so dl falls back to the track's vault key; licence messages name the track ("video 360p DV (592464997)").
Drop null keys from every licence, server and export source, not only Widevine. dl now finds remote sessions and imports through ServerKeySource instead of private fields.
Tracks with different KID sets still query the vaults at the same time, but wait for one licence lock and check the key cache again before they licence.