Brothers who don’t recognise brothers?
To be fair, they do not expect to find that the high official they see is anything other than a stranger.
Rashi adds a further reason to be fair to them. Since they last met, Joseph’s appearance had changed. He was no longer a 17-year old beardless youth.
But it seems that Rashi is not entirely satisfied with this explanation which, in fact, derives from earlier sources (Rashi’s genius is to see what the ordinary reader wants to know and to choose just the right answer from rabbinic sources).
In relation to Joseph, he goes on to quote a midrashic view that it was not just a matter of physical appearance but of attitude.
Even if they had realised that Pharaoh’s official was Joseph, they had never had any brotherly feeling towards him. They had always acted towards him without love or understanding as if he were not a member of the family but a complete outsider.
Even now, even if they did in fact see that this was their long-lost brother, they had no special feelings towards him.
Joseph, on the other hand, “recognised them”, i.e. bore no grudge and wanted to be friends.
It might cost him some of his public prestige to acknowledge that his family came from elsewhere and that he was not a full Egyptian, but he was prepared to pay the price.