It seems that Tor does not have a fallback. Although following paragraph can be found under Tor ChangeLog:
o Major features (client resilience):
- Add a new "FallbackDir" torrc option to use when we can't use
a directory mirror from the consensus (either because we lack a
consensus, or because they're all down). Currently, all authorities
are fallbacks by default, and there are no other default fallbacks,
but that will change. This option will allow us to give clients a
longer list of servers to try to get a consensus from when first
connecting to the Tor network, and thereby reduce load on the
In reality it has not been implemented and only the optional command line option * FallbackDir address:port orport=port id=fingerprint [weight=num] can be used.
I also made a fatal mistake by looking only at the public Tor Authority list Atlas and Torstatus . In reality one of the Authority server IP has changed and could be found only from source code, 18.104.22.168 -> 22.214.171.124.
After adding that IP also to iptables Tor could not establish a circuit and bootstrap. So to conclude Tor does not have a fallback, at least to this date.