Play a video in mpv, enter full screen either by pressing f or by double clicking, and there will be no animation, but do it again and there will be animation, but also if you play a video in mpv, then move the window with the mouse there will be animation when entering full screen for the first time. This is on the latest version of kwin and mpv, it also exhibited the same behavior on tumbleweed too, I don't know if this a kwin bug or something to do with mpv
Actually this isn't just limited to mpv, the same thing happens with firefox picture in picture, activate picture in picture, double click to go full screen, and there is no animation, but double click the same window again to go full screen, and there is animation.
Cannot reproduce on Wayland in Plasma 6. Are you using X11 or Wayland? And what version of KWin or Plasma are you using?
The latest version of plasma, but it only happens on X11
Plasma version 5.27.7