Prefixes are not suitable for matching DXCCs, as they must include the first digit in the middle of the callsign too in order to match (e.g. JA1, JA2, JA3 etc.).
The easiest solution is to add a DXCC condition, open the menu, click the “Select All” button, and then uncheck only Japan. This will leave you with lots of non-Asian countries still selected, but that doesn’t matter if you also have a Continent = Asia condition.