As described, your partner’s lighting conditions are perhaps less than ideal. Your partner’s camera may be trying to compensate for artificially low light conditions - slowing the frame rate and/or increasing exposure; FaceTime software itself may have some impact upon performance in very low light. Alternatively your iPad may have an issue, but for diagnosis you will need to be able to consistently recreate the issue for the Apple technicians to investigate.
Be aware that your iPad LCD screen, in common with all backlit LCD display technologies, is incapable of displaying pure-black. In low light, low contrast conditions, you will see and perceive a “grey” image - this is part being due to the uniformly lit backlight shining through the dark pixels that you see on screen. This is a fundamental limitation of LCD screen technology.
If you a fortunate in having a fifth generation iPad Pro 12.9”, your miniLED backlit screen benefits from zone dimming. With zone dimming, instead of having a single uniform backlight, individual areas of the screen can be dimmed - improving LCD screen contrast in dark areas by reducing (or selectively turning-off) areas of the backlight. This newer screen backlight technology can improve the perceived depth of blacks or dark areas of the screen.
If you can recreate the problem, you would be well advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider - where you will be able to demonstrate the issue for the technicians to assess. If remedy is required, you’ll be able to discuss your available options:
Genius Bar Reservation and Apple Support Options - Apple
