FINDING THE ISSUE:
Some editors complained that they were setting the inline style to some words in the richtext and publishing the content, although their inline styles were not being saved. They also said that doesn't happen all the time which makes the issue not easy to be reproduced.
WHAT WE HAVE FOUND SO FAR
Changing the inline style in the richtext attributes doesn't update the textarea of the richtext, as you may see in the following image:
If the editors click on the publish button, the textarea doesn't have the custom style information yet, so the attribute will be saved but the custom style is missing.
I have noticed that the textarea is updated when any object in the page gets the focus, or even when you hover the richtext area. Thus, to reproduce this error, set the custom style and avoid to hover the richtext area or click on any element which is not the publish button. You may see that the inline style will not be saved.
To illustrate that, please see the video in the link below: