Cannot Build Roads on Site-Graded Terrain

[Vectorus]

I need to play around a bit more to find out the exact situation where this problem appears. After relogging today, I found that a graded biodome yesterday now supported roads built within its graded area. Yesterday, they were rejected as being too far away from the virgin terrain level. (Is "virginity" recalculated at some point after buildings alter a planet's shape?)