It doesn't capture on the video itself on my MacBook, but you're probably seeing OLED black bleed. Not sure what the actual term for this is but if you're seeing the app tile's left and right sides doing funky things and filling in late, it's just the way OLEDs handle dark objects and is one of the few minor annoyances with this type of display. It'll also happen with dark gray on black background (like Apollo's night mode+true black options enabled scrolling through the list of subreddits) where the gray will nearly disappear while scrolling.
tldr: it's normal and within spec for the OLED display.