This thread is resolved. Here is a description of the problem and solution.
Problem: On the WPML > Translation Management > Multilingual Content Setup > Translated documents options, the "Copy from original language if translation language uses encoded URLs" option is selected, but the slug is not copied from the default language
Solution: Please, click on the WPML > Languages > Site Languages > Edit languages. On the secondary language row, look at the "Encoded URL" column and select "Yes". Click on the "Save" button.
Note that you need to resend the translation jobs or edit this manually for the old posts - already translated.
Relevant Documentation: https://wpml.org/documentation/translating-your-contents/using-the-translation-editor/
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