When undoing a split transaction that had splits spanning multiple accounts, the account balances aren't updated. The unsplit action doesn't update the account balances (it's assuming all splits were associated with the same account).
Steps to Reproduce:
  1. Setup Account A and Account B with balance of zero.
  2. Create a transaction (i.e. $100 in Account A)
  3. Split the transaction in 2 ($50 and $50)
  4. Set the account of one split to Account B and the other remains associated with Account A.
  5. Unsplit the transaction (notice the balance of the accounts doesn't change, Account B shows $50 and Account A shows $50, when Account B should show $0 and account A should show $100).