The JS code that handles disabling the Locally block anonymous users only checkbox does not properly disable the checkbox if the state of the Also block the given user locally on this wiki checkbox changes to be checked and the target field is not modified from the username of an account. When the target field is modified the checkbox is properly disabled.

Steps to replicate the issue

1. Open Special:GlobalBlock when logged into an account with the steward group and rights to locally block
2. Enter a username into the target field
3. Check Also block the given user locally on this wiki (or uncheck and then recheck the checkbox if it is already checked)

What happens?:
The Locally block anonymous users only checkbox is not disabled, as shown in the screenshot below.

What should have happened instead?:
The checkbox should have been disabled.