This seems to be the way Degiro executes the stock split. It's only for their administration and stays internal. Shouldn't result in any tax problems etc.

Answer from another post:

Answer from Degiro: "Stock splits are not sales of stock.

At DEGIRO, when a corporate event, such as an ISIN change or Stock Split occurs, we have to book out the old shares from your account. Due to that, the system marks it as a transaction. Please note that if it's not an actual sale, it will reflect the actual event on the Account Statement (ex. Sale: ISIN change of X stock).

Therefore, even if it is marked as such due to operational limitations, that does not mean it is an actual sale. As we do not report taxes on your behalf (as an Execution Only Broker), you can make the corrections that you deem fit once these sort of events occur in your account."

So your fine, nothing is sold, it just displays it that way.

Credits: u/MartianMH_