When removing an applet through its context menu, it disappears immediately whereas using the X on the applet handle, we get a nice animation first. The problem is that the "remove" action deletes the applet right away, whereas the X first runs the animation and then calls said "remove" action. Reproducible: Always
Both right click close and handle close seems not using animations or they are hardcoded and to fast - can't tell for sure - i set animations in 'Compositor' to 'Very slow' for investigating
Compositor settings don't alter Plasma
In case it helps anyone in the future, the issue is that the animation is defined in plasma-desktop/containments/desktop/package/contents/ui/ConfigOverlay.qml, so the QAction used by the menu can't use the same animation.
The relevant C++ code is currently in plasma-framework/src/plasma/applet.cpp and plasma-framework/src/plasma/private/applet_p.cpp