Field values not displayed on participant confirmation page

In the event registration workflow with "participant approval required", the participant is invited to complete his/her registration. A link is sent to them via email, which leads them to the registration form again where they are supposed to finally submit their data. Since they have filled in the form before, the values are to be pre-filled when returning to the form.

The problem I found is that some are, but others are not. Especially an advanced multi-select field inserted via a profile does not get pre-filled with the values entered before, although they have been stored in the database.

Since the workflow requires email functionality, I could not reproduce it in the sandbox.