Tax Treatment of US Stocks for Non-Permanent Residents in Japan

I understand that capital gains are generally exempt from Japanese tax while one is classified as a Non-Permanent Resident (NPR).

Suppose we have US stocks in our Interactive Brokers (IBRK) account, like NVDA, that were bought before entering Japan. If we sell just once during NPR, the gains are exempt. But what if we engage in multiple buy/sell trades of NVDA while in Japan under NPR status—would those gains still be tax-free, or could frequent trading trigger taxation?

Any insights would be greatly appreciated.

EDIT: I am from the UK. Bought the stocks there before entering Japan.