An objects space doesn't align with world space, just like a cloud doesn't unless in final. Probably the issue. Your mask is probably far below the object.
Depending on the objects location on Y the actual "0" of Y is offseted.
For me when I set a level, without it being final position, I can move a single object and the surface layer remains in position.
It's good to note that the scale of the altitudes seems to be somehow related to the object, and not world space. For example. The tree is only lifted 1 meter in this example. And the tree is only a little over 4m, but the altitude settings, as you can see, do not relate to these dimensions. The cut-off is 10 meters high, with a 5 meter fuzzy zone.