It is my opinion they put on the "happy" face. Being "happy" is no excuse for betraying people, also IMO.
Most of us noticed oddities. I, personally, saw strange behaviors but wrote it off to having trouble at work or losing his job. Mine, like yours, wanted to pretend I never existed. After lying to me, about me, gaslighting me, cheating on me, verbally abusing me and abandoning me, how could he live with himself unless he made me into a horrible person that he didn't want to remember?
It took a while, but hindsight being 20/20, I have the following:
He took everyone else's side but mine.
He would do what he promised for everyone except me.
He would pick fights in the car on the way somewhere, then walk into whatever event all charm and smiles while I tried to put myself back together.
He'd pick fights late when I was tired.
He'd come home late and not call. When he decided we "weren't together" but still lived in the same house, suddenly he called every night.
I noticed people looking at me oddly. Turned out he was lying about me to them, saying I was "bats#!+ crazy" and they were wondering what he was talking about.
He hid money.
He lost 3 Jobs in short order.
He started treating our then 16yo D and her friends like girlfriends (creepy as all get out).
He started spending money on fancy meals.
He forgot how to eat with chopsticks. And thought I didn't know how.
He took things I had done in my life and claimed those adventures as his.
He'd interrupt when I talked. Every single time until I told him to knock it off.
He'd do some lousy thing, then when I called him on it, blamed me for it. (Think of it like someone stealing from you and when you show them proof they stole from you, they tell you it was your fault for allowing them to steal from you).
Would say or do something, then claim he hadn't done it.
I'm sure there is more. I'm so far along I could not care less, so it's hard to remember. I do think we give them the benefit of the doubt because I believe we think they have the same honor, integrity and loyalty we do. And some people either don't have that, or lose it somewhere. My experience only.