Reviving a slightly older thread here. I had the same issue. It's a shame there just isn't more "stuff" out there yet about the Power Apps Creator Kit. This was the only post I could find on the subject of making a tab coming back to a default on page changes.
HVO, your solution came the closest for me, but I had a weird issue in that the first "click" after coming back to the page changed the tab, but not the selection border on the Pivot. I made some slight changes, following your lead.
On Screen Visible and Hidden, I set a variable to the tab I wanted as a default:
Set(varFormItemKey, "-Name of Default Tab-";
Then, in the OnChange of the Pivot Control, I set the variable to the ItemKey.
Set(varFormItemKey, Self.Selected.ItemKey);
That seems to do the trick for me. Coming back into the screen, the default Tab is set, but I can navigate through the other tabs fine as well.