"She and Jason were very much together as recently as this fall," an insider told People.
"Jason feels that the timeline that Olivia and Harry would like people to believe - that she and Jason split ages ago, long before she became involved with Harry - is simply not accurate.
"She began filming [Don't Worry Darling] in September, and by October, he began to get the impression that she wanted out. By November, they'd announced their split."
The source added: "But that's how quickly it happened, and none of it happened until she began filming with Harry."
On Monday, The Sun revealed that romance between Olivia and British pop star Harry has blossomed.
A source said: "They clicked immediately – they're both very artistic, passionate people and it was one of those things where straight away everyone could see they had a spark.
"It's no surprise something has happened, but now they're quite open about it and out and about as a couple."
In November, Olivia said she "danced with joy" after discovering that she would be working alongside the English boy band star.
She told Vogue magazine: "I did a little victory dance when I heard that we officially had Harry in the film, because we knew that he has a real appreciation for fashion and style.
"This movie is incredibly stylistic. It's very heightened and opulent, and I'm really grateful that he is so enthusiastic about that element of the process—some actors just don't care."
Olivia also described Harry - who impressed the director with his role in Dunkirk - as a 'modern man' and the spearhead of his generation.
She added: "It's pretty powerful and kind of extraordinary to see someone in his position redefining what it can mean to be a man with confidence.