Hi Andrey Rusovich
I'm not exactly sure what you're referring to. Are you talking about editing the wiki using our online editor or about something else entirely. Perhaps providing more details would help us figure out the error and fix it.
I'm assuming you're referring to the "commit message" field. What you're seeing is completely normal.
The wiki on Bitbucket is actually a repository. Each change you make to any wiki page is really a commit to this repository.
When you edit a page on the wiki and press save, the change creates a commit in the wiki repo with the commit message you mentioned before. When you press edit again, the Title and the Content fields reflect the current state of your wiki page, but the Commit message field is empty. The Commit message field is akin to a comment on the change to the wiki you are working on.
You should be able to see all your past commit messages when you click on the history button on your wiki page.