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Drupal theming has a trick to exclude CSS file of a base theme in a subtheme. You simply include the filenames of CSS files to exclude it in your info file. Then, if your subtheme doesn't have these files, the files are "overriden" to null state, meaning they are not included on the page.
With AdvAgg these files are still referenced in the page source, probably causing lots of issues (disabled page cache, empty aggregates). But I'm sure this is issue with AdvAgg because when I view the page with ?advagg=0 these files are not included.
I forgot to add this happens not with the base theme CSS files only, but with any overriden CSS files (including core modules').