This happens whenever you have multiple instances of a field on a layout and you put the focus on the field, e.g. with Go to Field. In this case, setting the selection puts the focus on the field. I believe it is the object stacking order that determines which instance "wins."
To control this, give each instance of the field an object name, and after the tab switch, have it go to the object on that tab. Then, when you set the selection, leave the target field option blank, i.e. don't specify a field.
Set Variable( $selectedStart ; Get ( ActiveSelectionStart ) )
Set Variable( $selectedSize ; Get ( ActiveSelectionSize ) )
Go to Object( "tabField2" )
Set Selection( Start: $selectedStart ; End: $selectedStart + $selectedSize - 1 )