GFM lists breaks with nested lists

GFM (GitLab Flavored Markdown) has at least one strange behavior from what I'm used to in GitHub one. In a list (ordered/unordered), the following markup...

1. An item:

    ```
    A code block
    ```

    A nested paragraph:

    - `an inline code`: some description.

2. Another item

...produces the output bellow:

1. An item:

   A code block

   A nested paragraph.

   • an inline code: some description in a nested list.

2. Another item

Note how the nested list breaks its parent. The 2. Another item becomes the first item on a new list.

At GH, the same markup produces the following (cropped screenshot):