Yes, I’d say that normally the understudy is supposed to play the part as closely to the original actor’s performance as possible. It sounds like the director is still present, and has seen the U/S perform. If that’s the case, and it’s not bothering the director, I wouldn’t worry about it. You may just be used to seeing the original actor, whereas an audience seeing just the U/S without a point of comparison would buy their performance. The U/S might also improve with more rehearsal, naturally making the nuances their own without changing the intentions. Is this understudy definitely scheduled to perform, and how many times? If the actors the U/S is playing with are bothered by the differences, perhaps they could work together to make a small adjustments. But if the gag is carefully choreographed, you may need to change more than a few things for the overall effect to be the same, and that process can be tricky without the director.