Link-Multiple field not displaying selected records

I created a Many-to-Many relationship between the Email entity and the Account entity.

This created a Link-Multiple field in the Email entity called relatedAccounts.
I added the field to the side panel by manually editing defaultSidePanel.json.
I'm able to select accounts to relate the email to and the selected accounts appear in the Link-Multiple field. However, when I reload the page, the related accounts are no longer displayed.
Despite the accounts not being displayed in the side panel, I'm able to apply a search filter to filter by related accounts and I can verify that the email_related_account middle table is properly populated. So the information is stored in the database correctly, but it's not displaying in the side panel.

How can I make the Link-Multiple field display the previously selected Accounts?