Ordering for event registrations in backend is lexicographically, not chronological

One-sentence description

Ordering in the events backend should be chronological when sorting based on a datetime field.

Current behaviour

When ordering on the "Registration start" field, for example, the registrations are sorted lexicographically, as can be seen in this screenshot:

Expected behaviour

When ordering on a datetime field, the registrations should be ordered chronologically.

Steps to reproduce

1. Create registrations for June 2nd and July 1st for a certain event.
2. Go to the admin page for that specific event.
3. Notice that the registration in July is shown before the registration in June.