Why are my Kindle highlights truncated or showing ellipses?

Publishers require Amazon to place a "copyright export limit" on the amount of highlighted text that can be exported from a particular book. This limit varies form book to book but is typically around 10% to 20% of the text. Once you exceed this threshold, highlights from the book will be truncated. 
You can see if this is happening to your highlights by visiting the Kindle: Your Notes and Highlights page and looking for the warning below:

user inserted

Separately, Readwise automatically appends an ellipsis to any highlight fragment not beginning or ending with punctuation. We do this for aesthetic reasons and to let you know there was more to the sentence.