It appears that setFocus does not work when the item is in a tab region. I have an on-page load dynamic action that sets focus on P2_FIELD2. This works fine.
If I put these fields inside a tab region, the set focus dynamic action no longer works:
The only way I can get it to work is to put the setfocus in a delay, which is hokey at best:
setTimeout(() => {apex.item( "P2_FIELD2" ).setFocus();}, 500);