Untranslatable image still presents changes during translation

Problem/Motivation

I hope I am not hunting ghosts because the steps to reproduce are quite simple for no one ever coming across this.

The source of this issue comes from #2644468: Multiple image upload breaks image dimensions, which fixed a bug with the image dimensions leaking.

However, it does cause an issue with multilingual sites when using the "Flag other translations as outdated" checkbox under certain conditions (see below).

EntityUntranslatableFieldsConstraintValidator compares each untranslatable field's submitted value against the stored one via FieldItemList::hasAffectingChanges(). Since image fields store the dimensions, we get the "Non-translatable fields can only be changed when updating the original language.".

- The previous fix should not be reverted - fixes a genuine issue as far as I can see.

Steps to reproduce

1. Install language and content_translation; add a second language.
2. Enable translation for Article. On /admin/config/regional/content-language, tick "Hide non translatable fields on translation forms" for Article - this is what makes isDefaultTranslationAffectedOnly() TRUE, which the validator requires.
3. Make field_image untranslatable. (note that you may simply create a new node type instead and a custom field anyway)
4. Create an article. Add an image. Save.
5. Create a translation for the article. Change the title (translatable). The image should not show in the form. Save. That passes fine.
6. Edit the original language. Click the "Flag other translations as outdated". Press Save (this translation). No need to alter anything

Expected outcome:
Entity is saved. No changes are detected.

Actual outcome:
Non-translatable fields can only be changed when updating the original language.

Proposed resolution

I am normally very reluctant in just adding a class in core for something that basic because as I said, it is weird to me that this has gone undetected (IF I am correct). But I think a new item list for the image field should be created that handles the values it needs to check for comparison.