When a file created with Prism 8 Windows is opened by Prism 7, Japanese characters can turn into rectangular placeholders.

The fundamental issue is that Prism 8 encodes Japanese with Unicode, but Prism 7 does not. So if you use the Arial font when entering some Japanese text when the Windows system is not set to the Japanese locale, you'll see Japanese in Prism 8 which automatically uses the "MS PGothic" font. This font is provided with Windows. When opened in Prism 7,  Prism 7 uses default Arial font that does not have Japanese character set. So the text shows as rectangles. 

To work around this problem in Prism 7: Select the text (rectangles) and change the font to one that includes the Japanese character set. 

To avoid the problem in Prism 8: When entering Japanese text, use a universal font (f.e. MS UI Gothic) that includes Japanese character set.