Hopefully Amazon can fix it. I too have this problem. Speak Screen reads the first page completely and after that the last paragraph on all other pages. I have low vision problems and as it stands now, I no longer can access my library of 100+ Amazon Kindle books via Speak Screen.
From reading the news Apple is revamping iBooks very soon (IOS 11.3) and to have Speak Screen work in the Kindle app would be to Apple's disadvantage. So now if you want books read to you via Speak Screen then you must use iBooks or PDF but not Kindle.
However you should be warned that Apple or Amazon could disable speak screen because they could not solve the problem. They could also stop supporting Kindle completely as well too. We are very lucky that the Kindle app still works on the iPad.
I am not surprised about the Speak Screen issue. In IOS 11 Apple also took away the ability to stream Apple iTunes m4a files stored locally on the ipad to DNLA media players that were fully supported in IOS 10. There were about 5 app developers who were effected and they had to remove that feature from their app. Apple and the app developers blame each other.