reason your move requests fail “above a certain line” is your navigation system’s settings. Whenever a pathfinding query is issues goal location is being projected to navmesh. projection has a limit and that’s why points too far above ground will fail as move goals. To tweak that go to Project Settings -> Navigation System and tweak supported agent’s default query extent (you might need to add a supported agent first, just add an entry to Supported Agents array). You should be fine with expanding just Z-size of extent.