Hi there, wonderwoman3.
I think I had the same experience. And I came to the conclusion that it was By Design. When you Split a User Story, the US number that everyone's memorised through the whole last Sprint continues to be the one to work on, carried forward into the next Sprint, and on balance I think that benefit outweighs the alternatives.
The Tasks that have been completed go into a new for-the-record User Story that remains in the ended Sprint.
Ticking the box to create a second new User Story as the parent of those two allows you to 'expand all' on that one and its Children to view the whole piece of work, which can be a very helpful perspective. (I used to hate it when a PO didn't tick that because he didn't see the benefit...)
Best,
--Nev.