So, i doubt they'll be talking about any heavy stuff if it just happens to came up, in my guess they're just talking and teasing each other right? As you mc divert the Convo to something else, anaise notices but kept quiet, while the other girl i assume don't realize and just go with the distracted Convo or something. The run away part i assume mc just divert not leaving the video chat?
Now the setting is where will the follow up Convo with only mc and Anais happen? If it's straight out in the video call but the third girl gone early, Anais can sorta try to ask and dig a bit, but if it has gone sometimes after that, in a sense i honestly think it will be harder for the continuation of the Convo to happen if there's no other trigger that make Anais confront mc about it. I mean, don't you think it would be kinda awkward if Anais just sorta straight out ask mc about her being uncomfortable talking about sex and romance (?) out of the blue?
So if you would like to follow with the conversation with video caht, the two of them can talk a bit about some random thing before Anais tried to ask what or why is mc being uncomfortable with the talk about sex and romance. Mc might be very uncomfortable, but as i assume they're good friend with each other, she will open up a little about how she doesn't feel anything like romance or sexual interest with others. It doesn't need to be direct, maybe jest her saying that "they weirded me out" or "i can't relate to it", in which Anais can come in with how she also felt like that once and being unsure of her own self in the past, and opening up about her being an aromantic and such. And then they talk about their frustration like you said? (What kinda frustration tho?)
I'm just gonna say that i don't know much about your mc and Anais, so it is just how stuff played out in my head, so just take this as a suggestion with a pinch of salt. A talk like this can be very delicate depending on their situation, as the "i don't feel love, and this should be wrong" might be something that your characters experienced, and maybe mc did and hasn't really came to terms with it (another assumption because teh way shes running away from the convo like you said), so just take it slow.
I mean, i read tons of romace book (and wtote about it too) but i don't feel a thing for other humans. I do love my fictional characters but humans is something else, but it's different with every person so might need some more questioning with other aromantic you could find.
If you decided that mc wouldn't be ready to talk to Anais in the video chat, you can make another setting where the two talk about it more on depth while they're facing each other or something.
I can't say i expresses what i meant clearly but i hope you get the gist of it? XD