Hm, I have several cases of a datagrid in a tab-container, without this highlight issue. So my first guess is that this is due to custom styling of your app. To see if that is so, create a blank app and see if you can replicate the same behavior.
And try the same in a Mx 9.12 version.
If it indeed is the styling, use your browser’s inspector (F12) to find out the class that is messing it up and ask you UI/UX expert to correct it.
Didn’t notice at first that the Tab Containers were also inside of a Navigation List. It is it Navigation List that was causing the issue. Every single item nested inside of the Navigation List is selected. I moved the Tab Containers out and not getting the issue anymore.