Typhoon heads to northeast Japan after causing some damage in south

By international (WMO) definition "Tapah" is not strong enough to be classified a Typhoon. It remains a strong Tropical Storm. Certainly it has and will have the potential to wreak havoc along it's path. So, be prepared as if it were a real typhoon. (News outlets tend to exaggerate.)

Every international typhoon watch organisation, including JTWC, have all called it a typhoon, so, there we go. It is a typhoon. @Burgan, you need to read up:

https://public.wmo.int/en/About-us/FAQs/faqs-tropical-cyclones

Every international typhoon watch organisation, including JTWC, have all called it a typhoon,

No actually the Joint Typhoon Warning Center called it a Tropical Storm, only JMA/JT called it a typhoon.

There is a difference in definitions, as there is no classification for a "tropical storm" in Japanese, the closest would be "ontai-tekiatsu" or tropical depression.

No, Brit is right. JTWC uses the names determined by the World Meteorological Organization (WMO) Tropical Cyclone Program. JTWC will add the tropical cyclone name in parentheses after the JTWC-designated tropical cyclone number only after the WMO-designated Regional Specialized Meteorological Center (RSMC) or Tropical Cyclone Warning Center (TCWC) names a cyclone. If the RSMC/TCWC has not yet named a cyclone, JTWC uses its TC number, spelled out, as a placeholder, i.e. "TS 16P (SIXTEEN). And since the topical cyclone was named, QED.

JTWC will add the tropical cyclone name in parentheses after the JTWC-designated tropical cyclone number only after the WMO-designated Regional Specialized Meteorological Center (RSMC) or Tropical Cyclone Warning Center (TCWC) names a cyclone. If the RSMC/TCWC has not yet named a cyclone, JTWC uses its TC number, spelled out, as a placeholder, i.e. "TS 16P (SIXTEEN). And since the topical cyclone was named, QED.

JTWC will add the storm name, however, until it gets large enough to be a tropical storm, it will stay a number, after that it becomes a "named" storm and tropical storms get named as this one was.

It was NOT a typhoon to the JTWC Tropical Storm 18W (Tapah) Warning #16 Final Warning

Issued at 22/2100Z

And by the way, tropical cyclones are in the Indian Ocean area, and the southern hemisphere, east of the international date line. Otherwise they are hurricanes.

https://www.metoc.navy.mil/jtwc/jtwc.html