I gather there have been similar issues in the past but this one seems distinct.
My mission "No Peace More Action" only allows one bridge officer to be selected. Upon zoning, the player IS able to request reinforcements but these reinforcements come out as unnamed redshirts rather than the default crew.
I believe the bug here pertains to my first custom mission map.
Upon review, it's the Chiron II Outpost (itself a redressed K7 as near as I can tell). I redressed the transporter room from that map to be a ship's transporter room.
Using this as the first custom map seems to only allow the player to select one bridge officer. (They can reinforce with redshirts to fill out the three remaining slots. But only one BO is selectable at first.)