Unfinished intersection

First I want to thank you for having a forum like this. I've found answers to a few questions over the last year and I am always impressed with the informative responses. Thanks!

We had an addition put onto our ranch. The builder and a helper did everything( including the roof) so it wasn't done specifically by a roofer.

The roof has been up since last fall. This weekend when I was cleaning out the gutters, I noticed an area where I can see the some of the wood trusses ( I think that's what they are) and exposed asphalt shingles at the intersection of the old roof and the new roof. I am concerned that water and ice will flow right into this opening and into the ceiling of the new room.

I went up there thinking I could figure out a way to lay down some flashing, but I can't figure out how to fix it.

My questions are:
Does this look like a problem that needs addressing?
If the answer is yes, would it be the builders responsibility to fix it?

Thanks for looking, I appreciate any advice or feedback.

Heidi