The bug

When you set CustomNameVisible to 1b for a mob it displays its default name. This is not the case for other entities.

How to reproduce

1. Use the following command

   /summon Cow ~ ~ ~ {NoAI:1b,Health:0.1f,CustomNameVisible:1b}
   

   The cow has the name "Cow"

2. Use the following command

   /summon MinecartRideable ~ ~ ~ {CustomNameVisible:1b}
   

   The name is not displayed

The reason

The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.

The method net.minecraft.client.renderer.entity.RenderLiving.canRenderName(T) should test if the value of the tag CustomNameVisible is true and if the entity has a custom name.

protected boolean canRenderName(T entity)
{
    // Replaced this
    // return super.canRenderName(entity) && (entity.getAlwaysRenderNameTagForRender() || entity.hasCustomName() && entity == this.renderManager.pointedEntity);
    return super.canRenderName(entity) && entity.getAlwaysRenderNameTagForRender() && entity.hasCustomName() && entity == this.renderManager.pointedEntity;
}