It's one of the problems of the Distance map, it may work in one place but produce wrong results when moving, it's not spatially consistent, unfortunately.
Sometimes, adding more tessellation to the distance object may help, but it can also produce more artifacts in other places. Using a chamfer modifier on the distance object sometimes helps, too, but this works best for hard-edged objects while yours is a chamfered box already...
3dsmax has a built-in map that works on simple primitives (TextureObjMask) but Corona doesn't seem to support it... But it doesn't work on a chamfered box, only a standard box.
I though I had a workaround by 'abusing' the UV modifier in Box mode for this, but for some reason it doesn't work the way I thought... sorry, I'm out of ideas.
Your best bet would be to try to 'fix' the distance object by either increasing tessellation or something else that would fix the calcualtion but I'm afraid it's a lot of trying until it works.
@Corona support - this is the problem we've talked about a few times. It really needs to be fixed, Distance map is essentially broken as long as it's not producing reliable results in all situations.
