NBT tag CustomNameVisible appears to be broken :

What was supposed to happen was :
That even when the ingame minecraft cursor was out of the entity's hitbox, the custom nametag of the entity was supposed to stay active/ON.
With CustomNameVisible:1

Instead :
The nametag does not stay active/ON when the ingame minecraft cursor is out of entity's hitbox.
Instead it acts as if CustomNameVisible was to false even when CustomNameVisible is true.

For example:

/summon <entity> ~ ~ ~  {CustomName:<customname>,CustomNameVisible:1}

Acts as

/summon <entity> ~ ~ ~  {CustomName:<customname>,CustomNameVisible:0}

Try out for yourselfs !

/summon <entity> ~ ~ ~  {CustomName:<customname>,CustomNameVisible:1}

e.g.

/summon Zombie ~ ~ ~ {CustomName:Zombie,CustomNameVisible:1}