Why does NavMeshAgent.remainingDistance return values of Infinity and then a float in Unity for unreachable destinations?

When I set an unreachable target position with SetDestination() for my NavMeshAgent and Debug.Log() the NavMeshAgent.remainingDistance every frame, I get Infinity for some of the path until it starts returning floats (21.21864, 21.0846, 20.95449...) until it gets as close to the destination as possible, which returns 0.

As of Unity 2019.3, NavMeshAgent.remainingDistance is still calculated only after the penultimate corner of the path has been reached, and the agent is traversing the last segment. Before that, remainingDistance will return infinity. Sadly, this is undocumented.

Here is a NavMeshAgent extension method to get the remaining distance at any moment, or any point of the path:

public static class ExtensionMethods
{
    public static float GetPathRemainingDistance(this NavMeshAgent navMeshAgent)
    {
        if (navMeshAgent.pathPending ||
            navMeshAgent.pathStatus == NavMeshPathStatus.PathInvalid ||
            navMeshAgent.path.corners.Length == 0)
            return -1f;

        float distance = 0.0f;
        for (int i = 0; i < navMeshAgent.path.corners.Length - 1; ++i)
        {
            distance += Vector3.Distance(navMeshAgent.path.corners[i], navMeshAgent.path.corners[i + 1]);
        }

        return distance;
    }
}

So instead of using NavMeshAgent.remainingDistance, you can use NavMeshAgent.GetPathRemainingDistance(). Be aware though this can be performance expensive depending the situation, so have that in mind when using it.

As for the second part of your question, we would need more contextual information of your setup, but sounds like your target position may have an offset towards the up vector, while the agent is constrained to the x, z plane, but this is only especulation.