The brightness bar in slo-mo is of no use.
Please fix the camera app completely.
This is normal.
If the slow motion video is set to 240FPS, it means that the shutter is 1/240 seconds and 240 shots in 1 second. Since the aperture of the camera is fixed, ISO will automatically adjust (depending on the strength of the light source will affect the ISO value) Video and photo are inseparable from the three elements of "aperture", "shutter" and "ISO". The intensity of the light source will affect the brightness of the video and photos. It is recommended to use slow motion video in a well-lit outdoor environment.