I was able to reproduce your problem - sort of.
1. Highlight last transaction and note balance.
2. Toggle sort arrow
3. Look at balance of highlighted transaction. It is different.
HOWEVER, in my test the transaction that was highlighted after the toggle was the one that had been highlighted previously BUT it had been moved down one space so the balance was different.
So if I have 10 transactions sorted ascending with #10 highlighted:
1
2
3
4
5
6
7
8
9
10 highlighted
and I toggle the sort to descending, I end up with:
9
10 highlighted
8
7
6
5
4
3
2
1
