Rear dashcam powered from rear USB socket

[sads45]

I have a small dashcam which has a USB-C socket. I wish to use this as a rear dashcam and thought I would simply run a long USB-C to USB-C cable from the rear USB sockets. However, when I tried this, the dashcam does not work. If I connect my phone to this cable, my phone charges so I know the cable is OK.
I have discovered that if I use a USB-C to USB-A convertor in the rear USB socket and then use a USB-A to USB-C cable to the dashcam it works. Anyone know why this should be the case?