As a workaround, I tried adding the discount information as a Plan Name Override instead of the Plan Price override . I set Override Plans Display to Yes and set Display Name and Description to Append, to which I added the text: - Bronze Membership 500 JPY Discount
However, this also renames the Current Membership plan so that it appears that the user's current plan is already the discounted plan, because the name and description override also seems to apply to the current membership in addition to the upgrade/renew options, so it does not seem to be a viable workaround...
Any chance this bug will be addressed in a near future update? It is pretty much the last issue to resolve before I can take this new site live.