That's not even how eminent domain works. You get the market value for the asset seized, not the delta in the market value for the asset remaining post seizure.
To the degree that this is true, that seems awfully unfair to people forced to give their property to the government without their consent. The government can just make you poorer overnight? That sounds bad.
If it was reversed and the landowner was buying the road to join two large lots together, you could easily see the road costing a huge premium on the open market.
But I agree the state didn’t write the law that way because it would be too costly.