Fixed - Why can't I display some Civi contact fields through CivCRM Entity in Drupal 8?
Graham - I've reproduced that. Not seeing the date of birth field either. Seeing lots of other dates though. Not sure what's going on. Could you please open an issue here: https://github.com/eileenmcnaughton/civicrm_entity/issues

Update: this is now top of the list for D8CE.

Updated: this is fixed now as of https://github.com/eileenmcnaughton/civicrm_entity/pull/190