When you change the zoom settings in Google docs, it changes the context it is sending to the Co:Writer extension. The new context is not reporting the proper cursor location to the Co:Writer extension causing the last letter to be deleted.
Solution: Use the Chrome browsers zoom function when using Google Docs.
Steps to reset Google Doc zoom and set Chrome browser Zoom:
Set Google Docs zoom to 110%, then back down to 100%
Click on the customize and control Google Chrome button in the top right corner of your browser
Adjust the zoom in the menu
Click back into the Google Doc
Refresh the page