I've not been able to find out what the problem is. One of the reason is that the Fusion 360 team has built tools to create all sorts of complexity, but has not built any tools whatsoever to manage that complexity.
As sad as that is, we can help ourselves at least a little my manually naming things
For example:
You have derived a Sketch named "default" into this assembly AAR ....
However for me and everyone who did not build this it is very time consuming to find what component that sketch is located in.

So perhaps give that sketch the same name as the component or use the component name as a prefix so the sketch in question can be located.

Edit: I "think" I found the source design for the "default" sketch. It looks to me that when the sketch name is updated in the originating design, the name is NOT updated in the target design. That is particularly disappointing as there is no other way to determine the source design.
I should be able to find the derive feature in the timeline, but that function also does not return a result!