With the system light theme, the dark variants of Themed icons are displayed. It's also worth noting that the background is lighter than before.

Android 15 (November) vs. 15 QPR1

When the system dark theme is enabled in QPR1 (December), you get light Themed icons. This is clearly a bug that Google should address in a future update. There doesn't seem to be anything users can do right now to fix it.

With QPR1 Beta 3, Google tweaked the color of Themed icons: With the system dark mode, icons were appreciably lighter. In light mode, the inner icon became slightly brighter. Google reverted the change in QPR2 Beta 1, so the original look is presumably the intended behavior.

Hopefully, Google will address this Themed icons bug with Android 15 QPR1's January security patch.