SUMMARY
When trying to drag a window to another monitor with the Overview effect, it is invisible in a couple of situations.

If that window has been scaled down at all - for example if you drag it up towards other virtual desktops, it gets a bit smaller. If you try to drag a window onto another display with it in this state it will be invisible until you let go of it, and then it will pop into existence on the other display. For example, a maximised window or just a very large window (such as Info Center

Sometimes though, the window will glitch and flicker when dragged between displays and then once it's fully on another display it will disappear. This is the behaviour when dragging smaller windows (such as Info Center) across displays and back again. This behaviour is a little less consistent.

If I'm dragging windows that aren't on the primary display, they are always invisible if I drag them between displays. For example moving a window from my middle display to one on the left, it is always invisible in the Overview effect until I let go. Sometimes if I'm dragging a window from my middle display onto my primary display, the window will show up on the primary display but under any existing windows, and only until my mouse cursor goes across the display "border" and then the window disappears. Other times it might flicker. It's very strange, seemingly inconsistent and "ugly" flickering behaviour but it's invisible for 99% of cases.

This bug is cosmetic to the Overview effect, the windows appear on the correct display once I let go and there is no issue once the Overview effect is closed. The problem is purely with regards to how the windows look when they are being dragged between displays, before the user lets go of the window.


STEPS TO REPRODUCE
1. Scale a window down in the Overview effect, such as by trying to move it up as if it were going onto a virtual desktop
2. Drag it to another display
3. Window will be invisible

Dragging to a primary display (the above still applies, this is for non-scaled windows)
1. Drag a window from one display to a primary display (if its scaled down
2. It will either be invisible or it will flicker

OBSERVED RESULT
Windows sometimes become invisible or are invisible with some very occasional flickering, when dragging them between displays with the Overview effect. 


EXPECTED RESULT
Windows should always be visible and smoothly scale and animate in the Overview effect.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.18.10-zen1-1-zen
KDE Plasma Version: 5.25.2
KDE Frameworks Version: 5.95.0
Qt Version: 5.15.5

ADDITIONAL INFORMATION
Tested on Wayland, have not tried X11