Update: I have reinstalled Kubuntu on the same laptop, but did not click "Install third-party software for graphics and WiFi hardware and additional media formats" during installation. I then manually added the proprietary Nvidia 390 driver after installation. When on prime-select intel, powertop now reports a discharge of 2.7 W, suggesting that the nvidia card is now turned off. The workaround was successful, indicating that the bug is in the installation process and not prime-select itself. Thanks for your help, Rouet and matlit!
Update: I have reinstalled Kubuntu on the same laptop, but did not click "Install third-party software for graphics and WiFi hardware and additional media formats" during installation. I then manually added the proprietary Nvidia 390 driver after installation. When on prime-select intel, powertop now reports a discharge of 2.7 W, suggesting that the nvidia card is now turned off. The workaround was successful, indicating that the bug is in the installation process and not prime-select itself. Thanks for your help, Rouet and matlit!