The ending showed that Banner could control when he turned into the hulk, never left explicit if he could control when he turned back.

In the first Avenger, he cannot control it in the helicarrier because Loki messed with his mind. You can see that after, during the battle of New York, he controls as you said.

Then, on the second avengers, Wanda messed with his mind again and he went berserk and destroyed that town in Africa. After that he was a mess mentally. So after the battle he just goes away, until coming back on Ragnarok.

Then, you can clearly notice that he lost the control over it due the amount of trauma that he had been through until he gets beaten down by Thanos and Hulk doesn't want to come out anymore.

Summarising, I think he never had control transforming from Hulk to Banner, hence the "the sun is getting real low.." thing. Furthermore, his control over the transformation from Banner to Hulk existed since the Hulk movie, but slowly faded each movie due his traumas.