Originally Posted by
freakazoid
.....Now my question is, how worried should I be about the missing leg? Whom should I contact about it?
None.

Your host airlines, US, has your complete itinerary. Other operating airlines, SQ and LH, only receive their respective segments, along with connecting segments as info segments. This is normal and there is nothing to worry about. This is the main reason why you must make any changes through your host airlines, US. @:-)