With dual monitors, changing one desktop will change the desktop of the other screen as well

Since I updated my Mac (I'm using a Mac Mini M1, macOS Big Sur 11.2.3), it has a strange behavior now. I have 2 screens. There is a mirror-like effect but not completely mirrored on both. I have the same amount of Desktops on both screens. If I had a new Desktop on one, it will had a new Desktop on the other as well. When I switch to an other Desktop, say I go to the 3rd desktop in one screen, it will switch to the 3rd desktop on the other screen as well. And last weird behavior: when I switch to full mode on youtube on one screen, the other screen switch to a full mode but with a black screen. However, I'm saying it's not completely mirrored because the 2 sceens are still different. I can have different activities on both screen besides the behaviors mentioned. Am I the only one who got this ?