When you click the add row button of a Repeater field inside a Tab other than the first one, it will display the repeater sub fields and then focuses the Tab to the first one. You then have to click that Tab again just to see the sub fields form.
I tested this and I can’t seem to replicate the issue.
I have a field group with various fields scattered across three tabs.
I created a repeater field (plus a test-sub field) on the 2nd tab.
I went out and pressed “Add row”, and I’m still on the same tab. Although there is no focus on the sub field, I’m not taken away from the tab the repeater is on.
This was tested on the latest stable version (4.3.4).