Since the update to Plasma 6 (complete reinstallation, EndeavourOS), the Steam client icon in the bottom right of the taskbar is only displayed as shown in the screenshot:
However, this only occurs if the original Breeze theme is used in light mode. In dark mode, on the other hand, the icon is displayed in white on a dark background. On the screenshot you can see a slight blue shimmer on the icon, which leads me to believe that the icon is actually intended in the original blue color. My graphics card is an RX 580 with amdgpu driver.