Around the 1:50 mark onwards is where I noticed it.
I just looked at your examples on both my LG CX OLED and my Samsung Q90R QLED (480 dimming zones), and I saw no artifacts like the ones you're seeing. It looks like macro-blocking, possibly caused by a low bitrate or compressed stream.
EDIT: Upon further inspection, there "might" be something wrong with your display as I cannot get those artifacts to reproduce anywhere, either on my PC or on the Youtube apps for my Smart TVs.