Doing a quick test here, I don't see that behaviour... works as expected... Using FileMaker Server 14 and Safari 9 and Chrome 50 on Mac OS X 10.11.4 (El Capitan).
You might want to check you are using a supported browser (particularly make sure you are not using Firefox), i.e.
Desktop browsers Mobile browsers Safari 8.x Mobile Safari 8.x on iOS 8.1 Chrome 38 or later Chrome 38 or later for Android 4.4.x Internet Explorer 11.x Internet Explorer 10.x (see deprecated technologies†)
If so, then maybe try a different layout theme and see if that makes a difference...
I'm also using Mac OSX10.11.3, Win 10, Chrome 50, etc. Seeing it on all.
Maybe the theme is the problem. Which theme did you test. Many thanks.
I tested using a new file using the default 'Enlightened' theme... Just a very simple file with a few parent records and with child records displaying in a portal...
For some reason the setting for the 'alternate portal row' was set to 'none'. When changed to white, the problem disappeared.