I agree that it's not possible for avoidants to be entirely normal for a year and then suddenly flip. When I was in a relationship with an avoidant, there were definitely signs of their attachment style. I just didn't see it because I didn't know about attachment styles and how to recognize them at the time. I also went through the abrupt and devastating breakup experience.

I suggest the book Attached by Amir Levine. It is a great resource to recognize and avoid getting into relationships with people with avoidant attachment AND how to find a secure partner. I used this book and am in a great relationship with a secure person now. Thank goodness!!