If your lights have built-in sensors, they will automatically dim to a preset level when first powered on rather than going to 100% brightness.
Here's why:
The sensors detect ambient light levels in the room and dim the lights accordingly to maintain the desired brightness. The light is programmed to turn on at a preset dimming level that is less than 100%. This prevents lights from being overly bright when first turned on. Photo sensors will automatically raise or lower the dimming level based on the amount of natural light in the room. So in most cases, the lights will not immediately go to maximum brightness when powered on or turned on from the app or a scene. The sensors modulate the dimming level upon startup for a more comfortable experience.
You can override this by manually setting the lights to 100% brightness after startup if desired. But letting the sensors control the initial dimming often provides a better experience.