Extra <br> tags might be added in Rich Text fields after saving changes in Experience Editor. The issue happens when having SC Hotfix-396633-1.zip and SC Hotfix-415404-1.zip or some other earlier cumulative hotfixes for XM installed. You can check this under WebsiteRootFolder\sitecore\shell\hotfixes in the Content Manager (CM) server.
For example:
<p>line1</p> <br> <p>line2</p> <br> <p>line3</p>
instead of
<p>line1</p> <p>line2</p> <p>line3</p>
To resolve the issue, consider one of the following options: