When hovering over the battery widget's charging icon, the green plug gains a white highlight over it. This is something that other widget icons lack, including the battery while not charging. So either that highlight is actually a bug, or all other widget icons are inconsistent.
Works as expected for me. Can you attach a screen recording that shows the issue happening?
Created attachment 176510 [details] Notice how the battery gets highlighted, but the other system icons do not (ignore the OBS ones).
The issue here is with Kirigami.Icon's highlight effect. It works as expected with a light theme and dark icons, but does not work as well with the reverse. This is because it tries to lighten colors, but the light icons are already pretty much as light as they can be — except for parts of them that are other colors, like the green plug in the battery icon, and the red slash in the mute icon. So only those parts get lightened.