Whether THEO started during the engagement or very soon after it ended is unknown, but I think 'the chemistry' between EO and TH certainly played some part in the engagement coming to an end.
But I'm not going to fault EO or TH for that - not that they'd care! We've posted about this before: even if her engagement was rock solid when rehearsing and shooting started - she's only 25, her fiancé is a 1,000 miles away, she's filming emotional scenes with a charming, handsome man who can work a British accent AND a Southern one. Then, because the town's so small and the cast's so small, they're going to cafes together and spas together and everywhere else together. It was almost inevitable under those circumstances that she would start to wobble in her engagement.
TH was coming off Bora Bora with The Arthy. He needed to rebound hard and fast after letting such perfection slip through his fingers.
So, yeah, the timing was bad and they didn't handle their attraction, entanglements, relationship in the best, most straightforward fashion. But - that's life and they're human. We here know he's no Prince Perfect, so this messiness (again, messiness = life) shouldn't come as a surprise.