For replicating the behavior:
- Open the close dialog by clicking on close
- Then choose option off-topic
- Then if you choose options like SSCCE problem, or minimal understanding, the vote to close button becomes active which is fine
- But choose Other (where you have to type custom message) option, now the vote to close will be disabled
- Go back to any other option like SSCCE problem, the vote to close button does not get enabled again.
When going in Other option in off-topic dialog and then reverting back to any other option the vote to close button is not enabled.
For me it always stays disabled for the first option I choose after Other, but after some other option is chose as @R.J. has suggested then the button gets enabled.