In Madrid Patch 2 on the Chrome Version 73.0.3683.103 (Official Build) the User ID gets auto populated with previously created user. And it looks like read-only field.
Safari does not auto populate, but provides a dropdown of possible user ids.
Firefox presents a blank form, as expected (no auto population).

Below is the screenshot of the field in chrome browser.

In Madrid Patch 2 on the Chrome Version 73.0.3683.103
1) Navigate to sys_user.list
2) Create a New User, say admin_test
3) Again click on New button to create a New user

Expected behavior: The user Id field should be blank without any auto populated value.
Actual behavior: The user id field contains auto populated value

This issue is under review. To receive notifications when more information is available, subscribe to this Known Error article by clicking the Subscribe button at the top right of the article. If you are able to upgrade, review the Fixed In or Intended Fix Version fields to determine whether any versions have a planned or permanent fix.