This looks like they have missed the movement joint and are now cutting it in.
The slots are so ties can be added to allow the movement yet tie the two halves together.
It demonstrates the lack of care and attention to detail Persimmon are known for.
In my opinion, this will never be as good as it would have been if constructed as the wall was built.
In fact, it may even be worse than leaving it with no joint as they are normally only required every 12m.
Please have your home professionally snagged and inspected before you legally complete!
I recommend Tim Fee 07768 900200 of Snagging Inspections.