Thanks for responding so quickly. I'm really enjoying the game.
Yes, I think it's the latter event. In a previous cutscene, both girls were talking and Girl 1 accused Girl 2 of dating me, then there was at least one more with Girl 2 asking what to do and I said I'd handle it.
When Girl 1 came up in the most recent scene, I answered with the option that I thought best fit what we had been talking about, so I was confused when she didn't accept that and gave me a 'that's not what [other idol] told me in our heart-to-heart message, liar' response.
I reloaded from a save just before the confrontation and tried other options, but every single one other than 'yes we're dating' gives me the angry liar response. I could try and get screenshots? Or just let it go, but I don't like the baseless accusation with no way out :P
For what it's worth, I'd been talking to Girl 2 partly because about her upcoming graduation and partly to find out who the bully in our midst is (it's Girl 1, but that doesn't have any bearing on this as far as I can tell, right?)