I think this is just a bug on the viewer, not the engine. The worst consequence is the guy who actually makes the assist not getting credit for it. There is a similar play when a player goes to the basket guarded closely by player-on-the-bench. (I wonder if in BB the benches are under the basket!)
The difference is that the player defending from the bench can't pad his stats in the league leaders by defending a play from the bench. If I'm not mistaken, assist leaders are tied slightly to merchandise income, which makes this more important.
What's more, we can't really know how effective our players are if the ones dishing assists (or even defending) are chosen at random, and can include guys who still have their warm-ups on.