@file docblock is associated as a comment to the following PHP statement

<?php\n
\n
/**\n
 * @file\n
 * TODO: Enter file description here.\n
 */\n
\n
/**\n
 * Implements hook_hook_info().\n
 */\n
function testmodule8b_hook_info() {\n

Gets me this:

array:1 [
  0 => PhpParser\Node\Stmt\Function_ {#219
    // SNIP
    #attributes: array:3 [
      "startLine" => 11
      "comments" => array:2 [
        0 => PhpParser\Comment\Doc {#212
          #text: """
            /**\n
             * @file\n
             * TODO: Enter file description here.\n
             */
            """
          #line: 3
          #filePos: 7
        }
        1 => PhpParser\Comment\Doc {#214
          #text: """
            /**\n
             * Implements hook_hook_info().\n
             */
            """
          #line: 8
          #filePos: 63
        }
      ]
      "endLine" => 19
    ]
  }
]

When the file has an import statement:

<?php\n
\n
/**\n
 * @file\n
 * TODO: Enter file description here.\n
 */\n
\n
use Drupal\Core\Routing\RouteMatchInterface;\n
\n
/**\n
 * Implements hook_help().\n
 */\n
function testmodule8b_help($route_name, RouteMatchInterface $route_match) {\n

the AST is:

array:2 [
  0 => PhpParser\Node\Stmt\Use_ {#170
    +type: 1
    +uses: array:1 [
      0 => PhpParser\Node\Stmt\UseUse {#228
        +type: 0
        +name: PhpParser\Node\Name {#214
          +parts: array:4 [
            0 => "Drupal"
            1 => "Core"
            2 => "Routing"
            3 => "RouteMatchInterface"
          ]
          #attributes: array:2 [
            "startLine" => 8
            "endLine" => 8
          ]
        }
        +alias: "RouteMatchInterface"
        #attributes: array:2 [
          "startLine" => 8
          "endLine" => 8
        ]
      }
    ]
    #attributes: array:3 [
      "startLine" => 8
      "comments" => array:1 [
        0 => PhpParser\Comment\Doc {#212
          #text: """
            /**\n
             * @file\n
             * TODO: Enter file description here.\n
             */
            """
          #line: 3
          #filePos: 7
        }
      ]
      "endLine" => 8
    ]
  }
  // ... etc

In both cases, the @file docblock should be treated the same in the AST.

Start by reproducing the two parsing examples in the issue and compare the resulting AST comment attributes for the @file docblock. Trace the parser entry point and comment-association logic involved; done means the @file docblock is represented consistently whether or not the file contains an import statement.