According to https://docs.microsoft.com/en-au/dynamics365/marketing/dynamic-email-content#assist-edit, Field values from lookups and related tables aren't shown in the Preview tab of the designer, or in test sends. Likewise, for-each loops aren't rendered in previews or test sends. To test your related-field expressions and/or loop functionality, set up a simple customer journey to deliver the message to yourself.

The step to explicitly create a user journey just for testing dynamic expressions via the assist-edit is totally user-unfriendly and adds additional effort for the end user. Creating a special journey seems unnecessary. Why do you have previews or test sends anyway if a customer journey is still necessary?
Can the testing for dynamic expressions be included within test sends in the ‘Marketing emails’ entity?
Category: Email
Ideas Administrator

Thank you for your feedback.

We released this functionality (test sends with dynamic content) as part of the wave 1 release 2020. It is documented here: https://docs.microsoft.com/en-us/dynamics365/marketing/email-preview#send-a-test-message.