The icon that Plasma uses for Bluetooth in the system tray comes from the SVG file preferences.svgz and not from network.svgz. Other network-related icons like Ethernet or WiFi are loaded from network.sgvz but not Bluetooth. It appears to me that the icons in preferences.svgz were intended for something else but they are being used instead of the Bluetooth icons in network.svgz.
No longer an issue in Plasma 6; now all icons always come from the icon theme, and Plasma theme icons are no more.