The bizarre bug manifests after switching from Fusion to Breeze; the style that is available in Plasma systems (e.g. Kubuntu 25.10 or older).

Although only the top tool bar appears to exhibit the bug, the attached example (qt-tool-bars.cpp) uses four tool bars to make the issue more obvious.

The attached GIF, recorded in a fresh, uncustomized Kubuntu 25.10 installation, shows how all tool bars switch color as expected. However, after switching to the System style (which happens to be Breeze in Kubuntu), the top tool bar (and only the top tool bar) will no longer show the correct background:

The issue does not appear to affect the classic Windows style on Linux or the Vista style on Windows. There must be something special about the Breeze style that triggers the issue.