Created attachment 159366 [details]
screenshot showing what happens when resizing tiles

SUMMARY
When clicking and dragging the border between two tiles, only the active window* has the translucency effect applied to it; even if there are multiple windows being resized. 

*or if the active window is not among those being resized, the closest tile to it.

STEPS TO REPRODUCE
1. ensure the translucency effect is enabled via "workspace behaviour>desktop effects>translucency"
2. set up a tile layout with more than one tile using meta+t
3. put one window on each tile (doesn't matter which application) using shift+dragging
4. click and drag the border between two windows

OBSERVED RESULT
only the active window becomes translucent (if the active window was not one of the resized windows, the nearest resized window to it will become translucent) 

EXPECTED RESULT
When resizing multiple windows using custom tiling, all the resized windows should become translucent.

SOFTWARE/OS VERSIONS

Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.25.4
KDE Frameworks Version: 5.105.0
Qt Version: 5.15.9

ADDITIONAL INFORMATION
I am not entirely sure whether the bug is still present in the latest builds, (I have not been able to test them yet).
I am also not entirely sure whether this is an issue with the effect itself, or the tiling, or the way that kwin handles windows and resizing in general -- I don't know much about the implementation of any of them.