It's not a bug per se. There appears to be some sort of a cap (apparently not very soft) for arena income. It depends on several factors, one of which is very likely the league level. Assuming all that true, you have obviously hit the cap for the total arena income or the bleacher income. It may not be a hard cap, but the threshold is significant enough that you have not been able to attract a single more fan for three games.
Personally, I agree it looks like a bug. It would be cool to have some minor randomness in those numbers. Perhaps you could have sold 15950 bleachers in the first game, 15948 in the second, and 15953 in the third one. The effect on income would be minimal, but it would at least not look so weird.