I think you may mis-understand the operation of that hydrant.
But, .. I'm in SC, and where I'm at the park has a warning that water pressure sometimes hits 120PSI.. IF this is the case where you are,, They may have added a regulator.
Regulators come in basically 3 sizes
The most common RV water pressure regulator operates exactly as you describe, Good pressure for the first little bit (As the water heater back flows into the cold line) then it tapers off.
The "High volume" regulators are not much better.
A good 1/2 inch or larger WATTS, gives full flow.
And of course they may have simply added a flow restrictor.
Like when I had the bathroom re-built in the Sticks and Bricks,, They put a flow restrictor in the shower, A washer with a hole in it that .. Well I can pass water faster than it did (or so it seems).. i removed it.
Home is where I park it!