When I create an automation to simply turn on a Hue light, SmartThings sets a default color and brightness level, even though my automation does not have the option chosen to set brightness, color, or color temperature. So if the light was last set at dim blue, it will incorrectly turn bright white if turned on by the automation.
When the automation is configured to just turn the Hue light on, it should just turn it on, and not change color and brightness levels back to default bright white values.
SmartThings is completely capable of just turning it on without changing brightness and color values; if you go to the Hue light’s device page in the SmartThings app, you can can turn it on and off WITHOUT it changing the brightness and color values. This is the way it should work, and the way I would expect it to work in an automation if the only action is to turn the light on.