Although I am not qualified to say what could be happening, I can say that I can relate to the frustration. Some questions if you don't mind.
Were you following maintenance and gaining? I know you were calorie counting but were you separating the fats/carbs, eating a big breaky, and having a fun day?
What were you doing before when you were able to keep it off for a year until the holidays? Have you noticed any changes that could contribute?
Do/did you have a coach?
I often wonder about the long-term impact on future weight loss with a diet like IP...can it cause permanent changes with metabolism that you will immediately gain if starting to eat a more SAD but then I hear well a lb is 3500 excess so there is no way but at the same time then any diet with calorie restrictions should cause losses right. I am with you, it is very confusing!
Let us know what you decide. Have a good day!