The lack of internal monologue sure makes it sound like a bug. You´ll probably have to wait for one of our resident code divers, like
ffive to confirm, though.
Meanwhile the only thing I could think of that would make somewhat of a logical sense for that situation to happen is if Lena is on Drugs, as sort of an override. Did you make her? If yes, do you have a save just before that? Try it without and see if it changes anything. But even if, the missing dialogue still doesn´t make sense.
As for the heart choices on Lena´s side. She will always stop herself from kissing Holly in that scene, if Ian is involved with Holly, regardless if they had sex or just kissed. You´ll stlll have to make that choice to access further heart choices and eventually kiss Holly later on.
Either way, I´m pretty sure Ian sleeping with Holly is the dealbreaker for the throuple in it´s current state, though. Don´t know if there might still be a re-entry planned for later but as is, you can only get there slowly, Ian kisses Holly but chooses not to sleep with her -> Lena forgives Ian a while later -> Lena kisses Holly at the end of Ch11.