Hi Lukas,
Welcome to Microsoft Community. The problem you encountered was reported by many users when Surface Pro 11 and Flex keyboard were first released. It was fixed later and no user has reported any type of problem recently. Therefore, it is recommended that you make sure your Surface has the latest drivers and firmware. You can check this through the Surface app or Windows Update, enumerate multiple times to strengthen the connection. Let me know how it goes, and we can explore other options if needed! Your SincerelyHahn. W - MSFT | Microsoft Community Support Specialist