Strange GraphQL type mismatch for non-plain fields in queryable interface

[seven-mile]

I have two types of documents to generate posts in my website. They are provided from two separate source plugins. So I declare them as 2 node types: MarkdownRemark and Typst. Both of them have the field frontmatter: Frontmatter. To be specific, this is the simplified version of the schema:

type MarkdownRemark implements Node {
  frontmatter: Frontmatter
  # other specific fields
}

type Typst implements Node {
  frontmatter: Frontmatter
  # other specific fields
}

type Frontmatter {
  title: String
}

Naturally I want to use queryable interface to model these common fields (frontmatter here). So I abstact out a new interface Post:

interface Post implements Node {
  id: ID!
  frontmatter: Frontmatter
}

type MarkdownRemark implements Node & Post {
  # ...
}

type Typst implements Node & Post {
  # ...
}

But when I execute the query allPost, there are errors about incompatible types of frontmatter:

Interface field Post.frontmatter expects type Frontmatter but MarkdownRemark.frontmatter is type MarkdownRemarkFrontmatter.
Interface field Post.frontmatter expects type Frontmatter but Typst.frontmatter is type TypstFrontmatter.

That's really weird. I already explicitly declared the field. However, the type of MarkdownRemark.frontmatter is MarkdownRemarkFrontmatter, which never shows in schema. I guess it is generated but have no clue in the large codebase.

Note that the difference between my usecase and the one documented is that I have a field with non-plain type. If I switch it to a plain type like String, it works well indeed.