You may have a small rock in the old water line leading up to the faucet.
I had one that that on Mercer Island.
When I cut the pipe, I found a small plastic ball that fit perfectly inside the pipe. When they tried to use the tub, the ball would slam up against the cold water port and shut that side off.
It may be bite the bullet time to see if you can find it.
The ports in the faucet can be very small.
If you are lucky, it may be in the cartridge itself.
That is where I would start.