So, I admit that I might have done something wrong during my upgrade to 5.2 (I managed to install 5.1.1 from the Kubuntu repo first...), but is this really supposed to happen?

Here I am hovering over the undo-button:



This also happens when I hover over New, Save, Zoom and Redo, and while these examples were all from KPaint, the Split-button in Dolphin has the same problem. The other buttons in Dolphin are continuously Oxygen however.

My Icon Theme is currently set to Breeze (I have never tried to change it).

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Edit:

Now I just tried to change it to Humanity, and then back to Breeze, and now all my normal and hover icons seem to be behaving properly.

So I'm not sure if I should even report this, but I'll leave it here for now, in case someone finds it interesting. I suppose it might be a bug, but I'm tempted to explain it away by my terrible upgrade path.

Looks like you've hit an icon cache problem. It's sadly rather common and the developers are already aware of it. Therefore it's probably not really worth reporting, as it's just one symptom of a bigger problem.