It is an illegal and degenerate position. It is no big deal. Besides Stockfish evaluates more drawn positions with other values than 0. Besides, this is only logical. Ideally all evaluations should be either 0, or +infinity or -infinity. All other values are inadequate approximations.
This is a well-known issue with computer evalutation. It is not known how to modify evalution so that both engine's Elo improves and it starts understanding such positions. The logic behind that is inherently hard and time-consuming, and implementing it somehow will decrease evaluation speed and still will not give enough improvement in position understanding.
