Mapped media fields are overridden with metadata on translation save

I have tested patch #62 with Drupal 9.5.3 and it successfully resolves the issue, that you cannot change the name ov a previously saved media entity translation.

The last submitted patch, 62: 2999370-62.patch, failed testing. View results →

The patch in #62 prevents the specific problem outlined in the description, but it introduces a regression. Specifically, metadata will never be mapped to entity fields, even if those fields are empty, unless the source field changes. This does not appear to match the original intent here.
It might not be a noticeable issue for most people because most metadata won't change unless the media source changes - e.g. if you edit an Image-sourced media entity and replace the image file. However, it's perfectly valid for there to be all sorts of other metadata that might be updated somewhere without the media source changing - e.g. a transcript on an externally-hosted video, a caption for an image stored in an external DAM, etc.
I wonder if we could restate the problem by defining the expected behavior as:

Available metadata should always be used to populate mapped fields when media entities are saved/resaved and the target fields are empty. When changing a media source field, we should expect any mapped fields to be updated with metadata-derived values, unless we are simultaneously providing alternative values for some of those fields.

I know it doesn't avoid the "I previously customized the media name, and I was hoping my custom name would persist after replacing the source file" scenario...but it seems to balance the concerns in this thread with the original intent.
Here's a patch that tries to put the aforementioned behavior into practice. If someone is able to look at tests for it or otherwise weigh in, that would be cool.

Updating patch to work with new media entity creation, improve empty value handling, and revise comments.

Please review.

The last submitted patch, 71: 2999370-71.patch, failed testing. View results →

+++ b/core/modules/media/src/Entity/Media.php
@@ -433,7 +432,7 @@ public function prepareSave() {
-          if ($translation->hasField($entity_field_name) && ($translation->get($entity_field_name)->isEmpty() || $translation->hasSourceFieldChanged())) {
+          if ($translation->hasField($entity_field_name) && ($translation->get($entity_field_name)->isEmpty() && $translation->hasSourceFieldChanged())) {

@Ajeet Tiwari - this proposal is the same as in #2 🐛 Mapped media fields are overridden with metadata on translation save Needs work . I don't believe this is good, because mapped fields that are empty should be eligible for population regardless of whether the source field is changing.

Hi, 62's patch has been re-rolled to work with 10.2.