The issue is present in betalabs.
It seems that the 'Save' button does not become active when a user is deleted from the Block list ('X' is clicked). Deleting a user from the list and then clicking somewhere outside on the page makes the 'Save' button active again.

Adding users does not have the issue - as soon as a user makes a selection from the list of suggestion in the Block list area, 'Save' button becomes active.

Another usability issue:

• if a username is typed (not selected from the list), the 'Save' button is active and can be clicked, but the username will not be saved.
• if a non-existing username is typed, the 'Save' button is active and can be clicked, but the user name is not saved. In this case, the message to the user, e.g. "the entered username does not exists" will be helpful.

In T173973#3546746, @Etonkovidova wrote:

Another usability issue:

• if a username is typed (not selected from the list), the 'Save' button is active and can be clicked, but the username will not be saved.
• if a non-existing username is typed, the 'Save' button is active and can be clicked, but the user name is not saved. In this case, the message to the user, e.g. "the entered username does not exists" will be helpful.

added to T174485: UsersMultiselectWidget should throw error if unselected text is present onSubmit

Change 375932 had a related patch set uploaded (by Dbarratt; owner: Dbarratt):
[mediawiki/core@master] Enable save button when removing item from UsersMultiselectWidget list

https://gerrit.wikimedia.org/r/375932

Change 375932 merged by jenkins-bot:
[mediawiki/core@master] Enable save button when removing item from UsersMultiselectWidget list

https://gerrit.wikimedia.org/r/375932