Well I think this was sort of answered in “Manhattan.”
It wasn’t that Rumple was a really different person when they got married and started planning a life. He was always kind and sweet–we see this even after the Rumple/Milah falling out that there’s a tenderness to him; and we see it in bursts with characters like Bae and Belle after Rumple became the Dark One–but there was also a hopefulness to Rumple. He was enthusiastic about going off to war, proving himself a hero, proving he was a good man and Milah certainly seemed to support that.
It was less about seeing something initially in Rumple that changed (like a latent character flaw) and more about Milah’s own perspective on Rumple’s actions on the battlefield (or lack thereof).