So likely mpm or routing issue then, can't do stopover at TYO between ICN and TPE but can do stopver at ICN between TPE and TYO. Is there a published rule on this?
See award description page
https://www.ana.co.jp/en/jp/amc/refe.../tk/usage.html
Incheon(zone 2) to Taipei (zone 3) requires 22,000 miles in Y.
Japan(zone 1) to Taipei(zone3) requires 23,000 miles in Y.
You can't select intermediate points which require higher mileage.