I read it on reddit that Jason didn't enjoy working with Heidi as in his opinion acting chemistry was missing. If I think about it that might actually be true but was probably contributing to why that relationship was so enjoyable to watch.
To be fair when she was first cast she was not supposed to come back with a fiancée storyline.
What a prick. The more I learn about the core Seinfeld group the less I like them. Googling Heidi Swedberg and looking at some pics of her it seems obvious why they didn't get along with her. She's a genuinely likeable and good-hearted person. At least that's my impression.