Laptops tested: Lenovo Thinkpad P14S Gen 3, P16 Gen 2 (separate power supply).

Android tested: Samsung Galaxy S10e,

Lenovo Display with hub P27h20: power and display works (for Android and P14S)

Thinkpad Thunderbolt (4?) dock: only display works (attached via DP) (for P16)

Lenovo USB-C 7-in-1 hub LC700: Power and display works (attached via HDMI) (for P14S)

In any case, none of the attached USB 2.0-devices work:

keyboards: Logitech Deluxe 250, Logitech media keyboard 600

Mouse: Logitech M705 (several)

Headset: Plantronics C720M

Space Mouse; 3D connexion Space Mouse Compact

Intresting, I just ran some tests using both CAC-1529 & CAC-1531. And indeed my USB connected devices did not work. So I removed the extension cable, plug the dock directly in to the laptop I used. And the USB devices worked, then I hot-plugged the Extension cable and gave it a few seconds, and every USB devices connected came to life.
My Setup included power delivery to the laptop and a 4K60Hz display.
Can you check this?

I’m not sure I know what hot-plugged means, but I connected my dock directly to the deck and had the USB devices working correctly, then I put the extension cable between the dock and the deck while the deck was on and the USB devices did not work even after waiting for over a minute.

My set up has the dock connected to power supply, the dock connected to steam deck. and a wireless keyboard transmitter, Xbox controller and wireless headphone dongle connected to the dock. I can connect an external monitor 1080p monitor and Ethernet without any issues using the extension, but USB devices don’t light up. They do receiver power through the dock but they don’t “wake up” as they do when you turn on a computer.

Hot plug means that everything is on and add the cable to it.

1. remove extension cable
2. plug every USB device in the steam-dock
3. Connect Steam-deck without extension cord to steam-dock
4. turn Steam-deck on let it find the USB devices
5. When all USB devices are found, do not turn off the deck and add the extension cable
6. If above does not work or you have done this go to 7
7. Turn off Steam-deck add extension cable
8. Turn on Steam-deck

Solved?

I tried with a wireless mouse, and it worked for me with my setup. Could be a weird glitch caused by the Steam OS ran on Linux.

I have done steps 1 through 5 for a couple of days now and 7 to 8 too. I originally thought my dock was faulty so I went through all kinds of solutions that people had suggested for it.

I don’t have many USB C devices but I do have a phone, and that connects fine to my PC computer with any other USB C cable, so I can transfer files etc, but this extension cord again just charges the phone and it doesn’t appear on the PC.

So, for you the USB devices were not connecting when using the extension cable but then hot plugging after they had been recognized fixed it? For all tested devices?

Yes, I used a Dell XPS, a dockingstation CSV-1564, and the CAC-1529 & CAC-1531.
At first the USB devices did not work, but after the steps provided, both  wired mouse and keyboard worked. Then did the same steps with a wireless mouse and it also worked.

Quite an odd issue.

Have tried wired peripherals? Or adding wireless devices one at the time?

Long Term, they still work. Even after rebooting.