Estimand containing terms only from USA is not identifiable without Australia domain

[djinnome]

Running the USA, Australia conditional counterfactal transportability query is identifiable:

conditional_cft_result = conditional_cft(
    outcomes=[+Infected @ -Vaccinated],
    conditions=[+Vaccinated],
    target_domain_graph=observational_study_in_usa_graph,
    domains=domains,
)

Resulting in this estimand:

((Sum[A](PP[USA](A | V) * PP[USA](I | V, A) * PP[USA](V)) / Sum[A, I](PP[USA](A | V) * PP[USA](I | V, A) * PP[USA](V))))

Note that USA is the only domain that is needed. However, if I remove the Australia domain from the list of domains, then it is no longer identifiable:

conditional_cft_result = conditional_cft(
    outcomes=[+Infected @ -Vaccinated],
    conditions=[+Vaccinated],
    target_domain_graph=observational_study_in_usa_graph,
    domains=[domains[0]],
) is None
True