Resolved
Overview of the issue
Ordinarily, when you have a post with a featured image that is translated, if you update the original post and change the featured image, the featured image on the translated post is automatically updated.
With WPML 4.8 this may not work.
The problem occurs because of the new setting in WPML > Settings > Media Translation to “Automatically detect best options for translating image texts”.
With new installs this setting is active by default, and causes the issue.
If upgrading from earlier versions the setting is not active and duplicating featured images works as normal, so it is recommended you do not activate the setting while this problem is addressed.
Workaround
If the problem occurs it will be necessary to manually edit the translated posts to update the featured image until this issue is fixed.