If header colors are used in the color scheme and a window is active, invoking task switcher (e.g. by alt-tabbing and continuing to hold the alt key) causes the active window to show the inactive header color, even though the decoration color continues to have the active color. This is on X11.
Can intermittently reproduce.
Just tried and cannot reproduce on wayland.
This affects present windows, and other desktop effects, as well.
Wayland is affected for me too.
This is likely a KWin bug; moving there, since inactive styles being used during tab switcher can be reproduced with e.g. Fusion.
Same problem occurs if you invoke the overview effect and cancel it, at least on wayland: the focused window's header color goes inactive.
When I invoke the switcher the inactive color is used, but if I cancel it it correctly displays focused header color, which is better than what's going on with overview effect.