I'd like to thank all of you who responded. Here are some new
information:

The engineer hired by the builder had a visual inspection. He thinks
it is differential foundation settling that caused the crack. The
differential setting is estimated to be 1/8" and he said this kind of
difference is covered by the design and is normal(?). He said because
of elastic nature of soil initial load caused the crack and the crack
should diminish over time with the current load. When the load changes
(e.g. furniture, snow) it is expected that the crack should be
proportional to load. The foundation is structural sound to support
vertical weight

What are your opinions?