Annotate results change when filtering *after* the annotate

[karyon]

Publisher A has two books with each 100 pages.

Publisher.objects.annotate(Sum("books_pages"))

will give you 200 as the sum. However, adding a filter after the annotate

Publisher.objects.annotate(Sum("books_pages")).filter(books__in=Books.objects.all())

will give you 400. (apparently that's the correct sum 200 multiplied by the number of books of that publisher)

I understand that joins in annotates can produce incorrect results, akin to the one documented here: ​#combining-multiple-aggregations. However, the docs there say only "Combining multiple aggregations with annotate() will yield the wrong results", and here I'm not combining multiple aggregations. Furthermore, ​#order-of-annotate-and-filter-clauses says "When an annotate() clause is applied to a query, the annotation is computed over the state of the query up to the point where the annotation is requested.", which further made me believe this should actually work.