Import content: Imported content appears in 2 different locations

Description

Steps to reproduce:

1. Site A is configured to use site-specific assets.
2. Page 1 has English version only, and it contains 4 related content items (blocks and an image from both "For All Sites" and "For This Site" folders).
3. Export Page 1 with its related content items.
4. Move all related blocks and images of Page 1 to trash.
5. Import Page 1 to Site A with the Update existing content items with matching ID option selected.
6. A warning "Property 'EPi:SystemReference' does not exist on destination content type" is displayed, but there are still 4 content items imported.

Expected result:

All related content items of Page 1 are in the "For All Sites" and "For This Site" folders. They should not be in the trash.

Actual result:

• In edit view, all related content items of Page 1 still exist in the "For All Sites" and "For This Site" folders.
• All related content items of Page 1 also still exist in trash. They appear in both trash and assets folders. It is not possible to restore them from the trash or empty the trash.