DId you use Paragraph Styles?
Now I didnt I just assumed that it would paste ok.... I am in the process of you tube learning Paragraph styles setup. That was probably my mistake...
Producing good reflowable text ePubs in not intuitive.
Take this step-by-step online course and it will explain how to successfully create ePubs and how to subsequently also convert them into mobi files which you can uploaded to Amazon to be read on Kindles.
And, of course, you can post specific questions on this forum.
I believe you can get a free week trial.