This seems to be caused by the bugfix for: https://jira.atlassian.com/browse/JRA-40310
In 6.3.9 and later, you can no longer add a comment in a transition screen if the destination status has jira.permission.comment.user set to false or denied. The message below is displayed in a red banner when you try to transition the issue:
<username>, you do not have the permission to comment on this issue.
This should not be the case because the comment permission property should be based on the CURRENT status of the issue. To put it in simply, the transition was not yet executed so adding a comment should be allowed.