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.
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.
