I was in class and took a picture of the slides with my phone (iPhone 13 if that helps) but the first one (I zoomed in a little on that one) turned out as shown while the next one was fine. The two figures in front are probably my classmates sitting down but nobody was standing in the back so why are there blurry spots there and why did this happen when I’ve never had a problem with my pic before? I took more later and they were fine too. Thanks!
This phenomenon entirely depends on how backlight of the LCD works. The brightness of LEDs is controlled with very frequent pulses (Pulse Width Modulation) which are mostly unnoticeable to humans. PWM works at fixed frequency and the duration of the pulse is what defines the brightness. Low PWM frequency is cheaper in production.
If your shutter speed is close to that pulse duration you will either get varying object brightness or even a completely dark screen.
The light being slightly visible on the edges is consequence of shutter acting gradually across the image (rolling shutter), so top and bottom of the screen were photographed at different moments.
You may observe similar phenomenon in other cases where LED lighting or gas discharge lamps are used.