You're right, the trigger is committing, i saved before telling Jeremy what i think of Lena and i just tried it and told him "no strings attached" but i also told Alison that i slept with Lena before and Alison didn't say anything about fucking around or a threesome.Okay that's interesting. In my Main Playthrough my Ian has a casual Sex Relationship with Lena and is going full Alison. In this Scenario Ian also told Alison in the Club that he and Lena slept together but unlike in your Playthrough in mine Alison mentions nothing and I can progress the Trip with her in the best possible way...Why is this so different? Could it be that your Ian kind of commited to Lena or told Jeremy he has Feelings for Lena? Because like I said before in my identical Holly-Playthrough Alison also suddenly starts with Jeremy and Threesomes out of nowhere so maybe it not depends on Ian fucking around but more if Ian commited to other Options like Lena or Holly?
But one way or another something is buggy about this espescially with the Threesome Lines.
i cant blame her for this bug, look at how many variables, triggers and diffrent scenarios are there just in Alison's route lol.