Sadly that storyline bothered me a bit too, especially since none of the other girls who had known her for far longer than the MC could even believe that she would do that since she's always been aggressively straight. Even the idea that the other girl, "Pam" more than likely charmed her in some way was more believable to everyone than the truth that she just woke up one day, obsessed and heal over heals in love with a girl she's known for less than a week.
It's kind of a fucked up way to tie up her story since even before she left, she was fiercely protective and possessive of the MC, jealous at the idea any of the girls would use the time she was away to steal him away from her. Not only did I find her offer for him to run away with them both disingenuous, even during and after their threesome, but it seemed like despite all the times the MC and Lizzie hooked up, she was more emotionally connected and concerned with pleasuring Pam over the MC, who came across as little more than a living/breathing sex toy that is there solely to get them off. Honestly, part of me wishes there was a path to bring her back to the mansion where the longer she was apart from Pam, the more she reverted back to her old self, and that Pam had a hidden talent of spellbinding people and getting them to fall for her and desperate to do whatever she wanted.