Well, yes and no. It's not a bug, if that's what you mean. The message is only available under certain circumstances. Since I can't add spoilered messages to this, I'll wait for your confirmation before telling you where to look, if you want to know.
The message is from Tsubaki, the green-haired girl from the track & field club. You have to pick her as your favorite at the track to get her number/give her your number, and thus be able to get a message from her.
Quick disclaimer, though: It's a rather short message interaction for now. There will be more in the future, based on this, but yeah. May or may not be worth going through the trouble of going back to that decision just to see it. :)