It's a fare combination issue. I had a quick look at YVR-TPE fares to start with (Fare Basis Code NLYWCT for example), and they don't allow double open-jaws:

CO.COMBINABILITY DOUBLE OPEN JAWS NOT PERMITTED.

It was the same for the YTO-TYO fares I looked at.

When you choose a connection, the fare is able to break at the connection point instead, obviously with a destination loaded with fares that allow double open jaws. In summary, it is AC's intent, and not IT, if you can believe it.

Hope that helps,
bawm