I contest that this is terribly similar to what happened on Kyoshi because even then, Aang did invite her along. Katara just refused.
And that wasn’t the only time that Katara tried to sit things out while Aang was surrounded by fawning fangirls where he in turn tried to make sure she was included. Remember The Headband? Once again, Katara very much so kept to herself. Aang was the one who encouraged her to dance.
So I do think it’s out-of-character. I can understand Aang’s excitement to a degree, but the idea that he wouldn’t try to include Katara — especially when he knows what it’s like to be excluded (Bato?!) — is ridiculous, frankly.
I still maintain that she’s just grumpy that they’re not at Iroh’s place right now. Having a romantic evening in the Jasmine Dragon, just the two of them, with Iroh busying himself “elsewhere” for conveniently long periods of time…
Oh for sure I’d be pissed too XD