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Hi, I did a quick google search and found this website saying there are approx 32 people with this surname in Japan (not sure how reliable it is): https://forebears.io/surnames/hoshizuka This website also allows you to enter info about the surname.
I couldn’t pin down which area of Japan from just a few searches, although there is a company with a similar name to your Grandmother’s surname so that kind of floods out the surname related info.
Also if I’m not mistaken, Hoshi mean Star in Japanese?
Anyway, I’m sure this wasn’t much help but best of luck tracing your family’s ancestry :)