Cracks appearing in knee wall less than one year after finishing 2nd floor.

In early 2021 I hired a general contractor to finish the 2nd floor in my 1955 home. It had been unfinished since the house was built. It went off mostly without a hitch.

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Less than a year later, we see these cracks appear around the knee wall/ceiling joint. It has happened in two spots on one knee wall. The other knee wall was built the same way and we see no cracks.

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I have no issue filling this with...something... and painting it over, but is this a sign that something may be wrong?

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In the attached album I have a picture of both cracks. The framing is visible from the crawl space, so I took one photo behind where a crack is, and another where a crack isn't. The gap in framing is concerning, but I have no idea how to frame so I may be worried about nothing...

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I guess what I'm trying to figure out is if this is a small fix that I can take care of, or a large issue that I have to go after the GC for...

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Thanks for looking!

https://imgur.com/a/RuIF0nH