They are aware of this issue as there is another topic regarding the same thing here.

I talked about this with Castagana, he told me that this isn't a bug and it is meant to be. And maxorator added that you must have a direct line of sight with the player's head in order to see the name tag.

So I don't know if the developers consider this as a bug or no. I just hope they do consider it as a bug since it'll be better to keep up with the player's health instead of trying to guess it.