Bug introduced in 8.0. or earlier, persisting through 11.0.1.
I know this site is not fond of screenshots, but this is the only way I could think of to communicate this behavior. When editing expressions that contain strings with exponents, such as
Quantity expressions, the exponents can disappear.
This has been sent to Wolfram as a bug report but not yet confirmed. As mentioned in the screenshot, the workaround is to not attempt typeset exponents, just use the carat