If you have oxygen-icons installed, the tooltip for the battery monitor will use an oxygen icon even when Breeze is the chosen icon theme. To make it use the correct Breeze icon, you must uninstall the oxygen-icon theme, though this may be a dependency of some legacy KDE4 program you may use. Reproducible: Always
There's some icons missing in the Breeze theme there.
I'm having the same problem. The icon for 'Charging' is missing.
Battery monitor only shows generic battery icon in the tooltip now.