Steps to reproduce the issue

1. Install a fresh Joomla version 3.9.x and call the super user
   admin%
   (username with a special character, e.g. % )

2. After Joomla has been successfully installed, try to log in to the backend using the new username "admin%"

Expected result

Actual result

Access denied with this error message:
"Username and password do not match or you do not have an account yet."

System information (as much as possible)

Additional comments

When trying to add a new user with special character in the username in the backend, you get an error saying:
"Save failed with the following error: Please enter a valid username. No space at beginning or end, at least 2 characters and must not have the following characters: < > \ " ' % ; ( ) &."

To me it looks like joomla is using different filters for username.