When I open an action card, the last email in a thread is expanded (and can be collapsed manually by clicking the sender icon). If there are more than two emails in an attached thread, you will only see the first and last one, the ones in between are collapsed and can be expanded by clicking the field "2 emails" / "5 emails" /...

However, once you have expanded all emails in a thread, you cannot collapse them again. You need to close and reopen the action card, which is rather inconvenient.