Probably you never had the 300k, you can go through you transactions and check them 1 by 1 to see where the difference occurs.
The total balance is always the correct one, you can also check it under "coin control" that's always correct.
There are 2 scenarios where this can happen:
1) You did some transfers between your own accounts in very rare cases this can cause your transaction grid to register something wrong.
2) With older wallets that were created before the first pandabank it can potentially happen because of the accounting system that was used before. But again there were no coins lost obviously.