Here's how to reproduce this bug:
1. Add a Comment to an existing Jira and include an image citation with a width=100% attribute, such as: !MyGraphic.PNG|width=100%!
2. Click Save and notice that the image shows full width, as it should. (Great feature, by the way).
3. Click the pencil to edit the comment. Make any type of change (but don't modify the image) using the Visual tab.
4. Click Save and notice that your image is now a thumbnail.
5. Click the pencil again and go to the Text tab and notice that Jira removed the "%" character after the "100". You can reinsert the "%" to fix things, but the problem will recur every time you edit the comment using the Visual tab.
Thanks in advance for fixing this!