It's a backflow - water in the inlet side will flow. Reverse the flow - trying to flow water in from the outlet - results in no flow.
I removed the backflow and tested with a garden hose.
The backflow has a spring to help the check valve. In past, I've had plumbing debris lodge on the spring side of the check valve preventing the check valve from opening. That was not the case this time for me.
If you can get a helper, you can check for flow all the way to the spray head by removing the waterline at the spray head and routing it to a bucket while the helper operates the water supply into the flush connection on the other side. A good water flow into the bucket means everything is good to that point.