Rendering ん as "m" before b/p/m is standard per Traditional Hepburn romanization, one of the most popular such systems. C.f. "Shimbashi", "kampai", etc. It's

Rendering ん as "m" before b/p/m is standard per Traditional Hepburn romanization, one of the most popular such systems. C.f. "Shimbashi", "kampai", etc. It's not an ad-hoc thing being done for "tempura". https://en.wikipedia.org/wiki/Hepburn_romanization https://en.wikipedia.org/wiki/Hepburn_romanization