Kde bundled "morphing popups" *IS* broken
This is because the crossfade effect does not work as intended since long time.
There is an open bug report here:
https://bugs.kde.org/show_bug.cgi?id=435423
...and here is a video showing the problem as of kwin 5.22.5: https://bugs.kde.org/attachment.cgi?id=140544

In the past i made this effect:
https://www.pling.com/p/1511353/ that strips the texture crossfade part from the original effect, but still gives you morphing and sliding shapes.

Now the upstream bug is partially solved, in the sense that the source texture is actually retrived in some circumstances.
This effect ensures that the crossfade between popups or notifications is activated only when there is a valid source texture to fade from.

Beware that this effect will work correctly starting from kwin 5.22.5 ; however installing and activating it now is harmless, and will work more or less like the actual morphingpopups.
Also, disable the previous morphingpopups effect when activating this.