Use function field on period domain
The account period model has a domain to restrict the periods to the current company in the context. It uses the fiscalyear.company field to filter it. Indeed the period has a company functional field (which returns the fiscalyear value). Using this field we get the same record result, but then the company field is hidden on the clients thanks to domain inversion.
So I think its better to use the functional instead of the fiscalyear join.