When using the review queues, the disabled button has become invisible:
For example, this is how it looks like for Stack Overflow:
This issue is maybe related to Buttons (white-on-light grey) look like they are inactive; it also looks similar to my previous report Disabled button not visible in flag dialog which has been marked status-completed. Here is a similar issue on another site.
EDIT: there's another site in the network with this bug, so I've posted a bug report on the main Meta.