this is not related only to grid-inside-grid scenario (HierarchyChildTemplate), but also for a flat (classic) grid control. If you have a GridView and any other control (ex TextBox), you will notice that if you move focus from GridViewCell to TextBox, both
This problem is still not scheduled for fixing. I raised its priority though.
The status of this issue isn't changing for a long time. Is there any chance that the bug will be fixed?
Cannot identify the current focused cell when a row from both the parent and the child GridView is selected