Good, then we should be on level ground then. Everything I've bolded in the first paragraph, I agree with. The thing is, the game does not need the model to be present on the screen in order to know the height or scale of the target model. Likewise, I find it unlikely that the game is targeting a model at all, but rather an object which is represented by the model and thus is not dependent on the visibility of the model itself.
I believe it is significantly more likely that this was an error avoidance method rather than a necessity by design. We lose targets on blink, not because the model disappears, but because it is safer and easier to say "Lose all targets when a model is set to invisible" than it is to say "Lose a monster/NPC type target when it despawns or dies, but not when the player blinks, and also do not lose target on players when they blink, etc etc etc". The latter can be done with some work, of course. The first is just easier. When the functionality that the players want requires the latter, however, I think it becomes something worth looking into at least.