This bug has been reported several times before and even was marked as fixed here (MC-57516) but the behavior is still not how it was intended to behave.

Original the CustomNameVisible tag allowed a Custom mobs name to display over the head of the mob even when a player does not hover over it. This does not work for mobs anymore, only for other entities.

This feature was often used in custom maps and will break them in this update because this feature does not work as intended.

For more documentation see:
MC-57516
MC-63118

This is not a duplicate bug report as the previous reports have been marked as fixed and it is still not working in the most recent version. This issue has to get addressed another time!