Well, obviously somebody in design did not expect that shims beyond a certain thickness may sometimes be required...
It clearly is a quality issue if (to get the job done...) workers in final assembly just force the parts together with shims that are too thin for the tolerances.
But it is a design issue, if you do not correctly predict manufacturing tolerances and don´t plan for them to happen and to be accounted for.
The seamlöessly wound barrel design does not allow to correct the inner diameter in production. It is a nice design with respect to strength, weight and fatigue performance, but a nightmare to deal with tolerances. And if you plan for thick shims, you lose the benefits again...
It was a wrong prediction of manufacturing tolerances that can be met with this type of production. It was lack of composites experience. Partly this is a natural learning curve (after all wound barrels are new), partly it is ignorance not to use the +40 years of experience from glider and small aircraft composites production (which does not exist in the U.S.)