I don’t have your account id so I can’t tell for sure, but this is what we think happened:
The Stadia transaction was initially authorised as a £13.49 debit, which triggered a £0.51 round-up in your savings pot via the coin jar feature.
When the merchant cleared the transaction, it became a credit for £13.49, while the round-up stays unchanged.
The feed behaves as usual with rounded up transactions: we show the total (+£13.49 - £0.51 = £12.98). If the transaction stayed a debit, it would have showed -£13.49 - £0.51 = - £14 , rounded up to the next pound as expected. In the transaction detail we show distinctly the real transaction amount (+£13.49) and the round up amount (£-0.51).
I’m sorry for the confusion but for a transaction to go from debit to credit when it’s cleared is highly unusual, I have spoken with few product engineers and none of us had seen before.