I don't know for certain, but I would suspect that the J allowance should extend to cover the initial sector you were charged for.
What did it say on your ticket?
They do document that they extend domestic allowance to mixed Dom and Int journeys.
https://www.fijiairways.com/en-gb/fl...ge-allowances/
You can see this in the notes at the bottom of the page.
So you might infer that they could do the same for mixed economy and business journeys but you cannot assume.
How about calling up and asking for a potential booking similar to what you have just flown and ask the sales staff to confirm the luggage allowance?