The Hearthling goblin detection radius is triggered even when Hearthlings are mining far below goblins so the radius seems to spread out in a sphere, rather than on a reachable (pathable?) plane. Goblins can be on top of a mountain while the Hearthling mining well below them will panic.
It’s probably not a bug, since I assume detection is working as intended for now and is probably just a placeholder, but it’s something to consider upgrading in the future.
indeed … this will likely get balanced in time, but I agree it certainly needs to be addressed…
your workers become highly unproductive, running around through the mines, waving their hands frantically, while the goblins stomp around several feet above… 
There are a few places where the Y distance isn’t taken into account where it should, I assume these will be tidied up in time (alpha and all that). For example, try building a farm above a tunnel/mine. The farmer will continue to work the ground where you placed the farm, but the turned over earth and the food being grown will actually appear in the tunnels directly below that (at the lowest Y point that is available).