Recording time for UHG Resolution is limited in S9, S9+

When recording videos or using the streaming feature for extended periods, or when using features/apps that require more power, the device and its battery may temporarily heat up due to increase in battery consumption.

Using a higher resolution will result in higher quality videos, but it may cause the device to heat up and drain the battery faster. 
To prevent any device damages due to overheating, it is an intended normal operation that the device will stop recording automatically at 5 minutes (for UHD 60bps) or 10 minutes (for UHD 30bps).