Doesn’t work for me either. 
System information:
Operating System: Manjaro Linux
KDE Plasma Version: 5.24.2
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.2
Kernel Version: 5.16.11-2-MANJARO (64-bit)
Graphics Platform: X11
Graphics Processor: NVIDIA GeForce GTX 1650/PCIe/SSE2
Perhaps fixed in KDE Frameworks 5.24.3?
UPDATE: Found a solution by mistake!
Settings → Workspace Behavior → Desktop Effects → Window Open/Close Animation
Set this to “Fade”.
Set this to something else, and then click Apply.
Viola! 
UPDATE 2: It appears that in order to get “Fall Apart” to work, you simply need to change “Window Open/Close Animation” to something else, and then click Apply. This appears to “reset” a faulty setting that “blocks” the “Fall Apart” effect.