It's normal behavior in SharePoint, by default SharePoint URL label only shows around 50 characters as label, if URL exceed as in your case it replace all remaining characters with dots "..." It's nothing to do with sub-site or "Site Settings>look and Feel" page.

It's normal behavior in SharePoint, by default SharePoint URL label only shows around 50 characters as label, if URL exceed as in your case it replace all remaining characters with dots "..." It's nothing to do with sub-site or "Site Settings>look and Feel" page.

https://aaaaaaa.sharepoint.com/Project/SubProjectSubProjectSubProject
SharePoint replace it like this
https://aaaaaaa.sharepoint.com/Project/SubProjectS.../