Field properties are not normalized or denormalized on entity reference items.

I think we're making exactly this change in #1927648: Allow creation of file entities from binary data via REST requests.

Free reroll.

I have found this problem to also obfuscate the title, alt, width, and height attributes on the ImageItem field type. This creates a blocker for management of image accessibility for headless use cases.

Let's retry those tests.

+++ b/core/modules/hal/src/Normalizer/EntityReferenceItemNormalizer.php
@@ -103,9 +120,15 @@ protected function constructValue($data, $context) {
+      $constrcuted = array('target_id' => $id);
...
+          $constrcuted[$property_name] = $data[$property_name];
...
+      return $constrcuted;

Consistent typo. => constructed

Note that this solution sidesteps Typed Data, so any modules dynamically generating API payload descriptions (Schemata) will not be aware of these properties.

Furthermore, the current implementation is adding the alt text and so on as properties, rather than fields, which makes for a "top-level" inconsistency in the JSON, since every other element is an array.

Note that this solution sidesteps Typed Data, so any modules dynamically generating API payload descriptions (Schemata) will not be aware of these properties.

That makes this approach a dead end IMO.

The example in question is a file reference field which is missing the "display" and "description" keys.

The solution in that case is to provide a FileItemNormalizer, because class FileItem extends EntityReferenceItem.

This patch/issue is proposing to modify EntityReferenceItemNormalizer, which is the wrong place to do this. We have #2825487: Fix normalization of File entities: file entities should expose the file URL as a computed property on the 'uri' base field for this.