Resolved
Overview of the issue
If fields are set to Copy-Once and the copied default field content is removed on a translation, the front-end continues to re-publish the default content. Instead, the translated field behaves as if set as Copy, and it is impossible to leave it blank.
Workaround
This is expected behavior that allows WPML to maintain compatibility with Advanced Custom Fields and avoid potentially conflicting translations.
You can leave a translated field empty by instead adding a space to those fields. This way, WPML sees the field as populated with content, and ACF sees it as empty.