Comment by Groxx
Surprisingly: USB-C cables do have an orientation. It comes up a lot with these kinds of female-to-female USB-C cable extenders: https://www.amazon.com/dp/B0CTT1FJL6 (I have not used this one, it's just an example)
>Important Note: Ensure your USB-C cables support video transmission when using this coupler for video pass-through. If the connection doesn’t work initially, try reversing the orientation of the cable’s plug to ensure proper functionality, as USB-C protocols depend on connector orientation.
AFAICT not all cables are like this, but quite a few are, and broadly it appears that it's the sockets that are reversible and are simply hiding this - cables often just use one side. So when you bridge two cables like this, you need to make sure those (unmarked) sides line up.
So I suspect one side of your connection is either damaged or cheap (and didn't fully meet the reversible spec to save money).
(but only suspect, I haven't found a way to fully validate this)
> it's the sockets that are reversible and are simply hiding this - cables often just use one side. So when you bridge two cables like this, you need to make sure those (unmarked) sides line up.
Thanks for explaining the unexplainable! Also seen with couplers and gender-changers for connecting USB-c cables.