Import Contacts with Email field issue
Yes it appears to be a bug.  You should file one. 

Here seems to be the conditions:

Importing update or fill, i.e. contact already exists
Has no existing email address or any existing has is_primary flag = 0
Has a street address or are also importing one

There seems to be no way via import to fix these or to import them correct.  Well, if you are also importing a street address, you can to do two imports, first just adding email address and second adding the address.  

I guessing the design is for any new additional email address imports to not make them primary (makes sense) and for some reason the existence of a street address makes it think there is already an existing email address.

Here is some sql that will give you back any contacts with email addresses that don't have primary one.  

select c.display_name,
       e.email,
       e.on_hold,
       e.is_primary,
       e.id eid,
       t.name,
       c.id cid,
from   civicrm_contact c,
       civicrm_email e,
       civicrm_location_type t
where  e.contact_id = c.id
and    e.location_type_id = t.id
and    not c.is_deleted
and    not exists 
       (select 'email primary'
        from   civicrm_email e2
        where  e2.contact_id = e.contact_id
        and    e2.is_primary)
order by 1