[Beemer]

I think the backfill was done about a day after the walls were poured

Concrete wasn't cured in the least and yet they backfilled. This is stupidity at work.

If possible, run a string-line along the top of the foundation wall that is cracked, just below the rim joist, from corner to corner to see if the wall is wowed inwards towards the house.

If it is wowed, you found the cause of the crack. Back filling against an unsupported, uncured foundation wall. This shouldn't be attempted before 7 days and even then, the sub floor must be installed before even a bob-cat comes anywhere close to the foundation.

The developer is at fault for putting the push on most probably. Of course, backfilling creates easy access to the building and thus speeds the building process up considerably. But at what cost if damage is done?