Hi,
Generally, the spotlight on the Tapo C320WS/C520WS will turn on automatically in the following scenarios:
Case 1. The spotlight turns on temporarily when the camera detects movement at night (dark environment) in Smart Mode.
Case 2. The spotlight turns on permanently when the Full Color Mode is selected in the camera's Night Mode Settings page, and the camera selects Night Mode on the Live View page.

@TTO121 
In your usage scenario, the spotlight works as it should if the camera works in the Night mode & Full Color Mode. If the camera works in the Night mode & Smart Mode, please confirm if the camera always detects events thus the spotlight is not turned off.

@Peter111 
Is your Tapo C320WS's hardware version V1? Please help check if the camera works in the Night mode & Full Color mode when the spotlight turns on.

You can try to change the camera's video mode to Day mode, or change the Night Mode Settings to Infrared mode, and then check if the spotlight will turn off.

Best Regards