GraphQL inline fragments not required?

Description

In GraphQL I seem to be able to access custom fields on entries without inline fragments. Is this a bug or a feature?

Currently this query returns the correct data without error:

entries {
  customField
}

However as far as I can tell, the inline fragment should be required:

entries {
  ...on section_entrytype_Entry {
    customField
  }
}

While I think this is currently a bug, it is actually quite useful as the site has many entry types with shared custom fields which would result in a huge query similar but multiple times larger than:

entries {
  ...on section_entrytype1_Entry {
    customField1
    customField2
    customField3
  }
  ...on section_entrytype2_Entry {
    customField1
    customField2
    customField3
  }
  ...on section_entrytype3_Entry {
    customField1
    customField2
    customField3
  }
}

Additional info

• Craft version: 3.5.14
• PHP version: 7.3.19
• Database driver & version: MySQL 8.0.20
• Plugins & versions: