Steps to reproduce the issue

Create a menu item type Wrapper >>> iframe
In the url field place a relative url (as per https://docs.joomla.org/Help5.x:Menu_Item:_Iframe_Wrapper#Form_Fields)

In the Advanced tab set 'Auto Add' to No
Give the menu item a Title and save

Expected result

The item will save and will appear in the front end

Actual result

The url is prefixed with http://http///
An error message appears
Invalid URL: URL schema is missing in http:///index.php/typography. Please add one of the following at the beginning: http, https, ftp, ftps, gopher, mailto, news, prospero, telnet, rlogin, sftp, tn3270, wais, mid, cid, nntp, tel, urn, ldap, file, fax, modem, git.
(I used typography as the target)

System information (as much as possible)

Additional comments

I tried by prefixing the relative url with http:/// as said in the error message and it saved but prefixed the url with http://

Looks like the 'Auto Add' setting is being ignored and the docs are incorrect?