It does not create new slots, it can degrade existing slots
The power pearl states:
You can use an action to say its command word and find a spell location.
Thus, you can only find a location already used. But:
If the occupied location was 4th level or higher, the new location is 3rd level.
So, if you choose to pick up a slot above level three, it is effectively downgraded to level 3.
There would be no benefit and a considerable loss to this persistence during a long rest. After a long rest, all your normal niches come back. There is nothing to "lose" at long rest – as Pearl of Power has never won you anything more than your usual build – what you earn temporarily is always equal to or worse than what you started.