The revision/change bar is appearing next to the page number if there has been a change to the numbering of the page.
This revision bar turns up next to the page number if there has been a page added before this page causing the page number to change or if changes have been made to the page before that they will impact the following page, for example, an added heading level.
If you want to remove this revision bar, follow these instructions:
Go to Settings → Documents → General → Tick out the "Show revision bar for page numbers" and then press Save