Hi and thank you for your report.
The reason why 5 or 6 subs were created is because of the minimum amount in JPY. It is around 50-60 JPY (depends on the JPY/USD exchange rate, should be 0.5 USD minimum), but your test plan was for 10 JPY. So instead of showing an error that the amount is below minimum, Stripe kept creating subscriptions until minimum amount is reached – e.g. 5-6 subs x 10 JPY = 50-60 JPY, which is the minimum amount.
I have created a new TEST mode plan, but made the price 100 JPY and now it’s working fine, please check yourself.
We will add the error message when creating a plan if the amount is below minimum.
Apologies for the inconveniences you had to face due to this and thank you very much for reporting it.