Here is what Phillips emailed me:
Thank you for contacting Philips Monitors Service and Support.
Please verify the following things about your set up:
1. The monitor has to be connected directly into your computer. If your computer does not have a USBC port that supports dp-alt mode, you will have to use two simultaneous connections: one for video (HDMI/DP), and one USBC from the monitor to USB to your computer. Please make sure that the USBC to USBA adapter cable you use supports power and video delivery.
2. If your computer or laptop has a built in camera, you may have to disable it from the computer's device manager.
3. Remember that for the webcam to be on, it has to be out and facing the front.
For more information about how to set up your webcam, please refer to page 11 and 15 of your user manual: https://www.documents.philips.com/assets/20230614/943b77569c5649d6bef1b021005a8488.pdf
Please let us know if the details above help with your issue.