As a part of moderation, some older posts are being locked to discourage reviving old posts and to eliminate spam attempts. The Message Boards disallows posting when the message is locked and displays a lock icon to indicate the lock.
Questions does not contain a lock indicator and also displays the answer box giving the impression that an answer can be supplied. When a user attempts to post an answer nothing happens (as the post is locked on the backend).
Similarly, a post without update permission also displays the same behavior of displaying the answer text box.
An example of how it could work is place a lock next to the title when a post is locked:
And disable the text box for posting an answer:
For more info see: https://github.com/liferay/liferay-community-site/issues/455