Well, I don't really know what more to say.
I was simply increasing the girl's love stat (3 for each a day) when, at some point, I realized the difference in their value, which seemed strange but why not? I may have done something wrong. So I continued the same, even started to increase Mandy's lust but then I saw Cherie's love stat was small again. At this point, I was scrutinizing the evolution of the stat. In the morning scene, the stat was reduced once more. I remembered I encountered a random event with her when going to sleep. So I decided to use the back button past that event and recovered what I had lost.
Then, I decided to try other random events and the bathroom scene generated that same issue.