If I call KUriFilter::filterUrl with an URL with a query containing two @ symbols, some of the special characters are encoded, which changes the URL. For example:

https://abc.com/?mail1=mail@xyz.com&mail2=othermail@abc.org
is filtered into
https://abc.com/%3Fmail1%3Dmail%40xyz.com%26mail2%3Dothermail@abc.org

I don't think that this should happen, as the original URL is valid.

I don't know whether this is a new behavior or not: I started noticing it a couple of months ago using Konqueror to open a link to a message in Google Classroom. I got the link in a mail sent automatically by Google Classroom, so I assume it's correct (and indeed it works in other browsers). However, after clicking on it I was shown a page claiming the URL didn't exist. I then tried copying the URL from the mail and pasting it in Konqueror location bar. After pressing return, I noticed that the URL in the location bar had been changed, with several special symbols (?, =, one of the two @) being replaced by their percent encoding representation. For example, the URL

https://accounts.google.com/AccountChooser?continue=https://classroom.google.com/s?email%3Duser.name@my.company.it&Email=user.name@mycompany.com
had become
https://accounts.google.com/AccountChooser%3Fcontinue%3Dhttps://classroom.google.com/s%3Femail%253Duser.name%40my.company.it%26Email%3Duser.name@mycompany.com