rendered entity usage in "add node" form for an entity ref field In one of my "add node" form, I would like to display the 'name' field of referenced content type rather than the title.

Let's say, Content type 'Car' contains a field called 'owner'  which is entity ref to content type 'person'.  'person' has a unique title but also has a field called 'name'.  

While displaying to the user '/node/add/car' form, I like to give the user a 'select list' for the field 'owner'.  I want to populate this field with the names of the 'person' nodes in the system and not their 'title'.

So in 'manage form display' of the content type 'car', I used 'select list' and in the 'manage display', selected for the 'owner' field, format of "rendered entity". But this displays only the title in the dropdown list in 'add/node/car'.

When that did not work, I tried using the new d8 feature,'display mode'.  I created a display mode called "nameview" and created a "nameview" display for content type 'person' which displays only the field 'name'.  I then used that view mode in the "manage display" for 'Car'. Still I get only the titles in 'add/node/car'.

What am I doing wrong? or is this a bug in d8?
What you configure in Manage display has nothing to do with what is shown in the form. That's not how it works.

You can do this using a view. In field settings, configure it to use a view show possible values and then configure that view to show the values you want to see.