Thank you for your post!
I'm not seeing any reports of this issue in FileMaker Go 15. I am also unable to reproduce any issue when tapping on a Portal Row in FileMaker Go 15 with an iPad on iOS 9.
efficientbizz offered good advice on checking the Portal Object's Graphic Attributes.
You might also see if this happens to you in a new database file.
Additionally, I am going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive more views and potentially more advice!