Topics sorting is impossible in forum moderation MCP tab.
This is due to this code https://github.com/phpbb/phpbb/blob/3.0.x/phpBB/includes/mcp/mcp_forum.php#L36-L37 (for 3.0) and this code https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/includes/mcp/mcp_forum.php#L40-L44 (for 3.1).
The code itself was initially added by Ludovic Arnaud on 28 Nov 2003 (see https://github.com/phpbb/phpbb/commit/a08d9e647300dfa70052245d7fd1cddacdbd0453#diff-f9ca89bd8b8c2301367e03006a78a3daR314) and currently seems to be irrelevant as the bug described in the comment (// Fixes a "bug" that makes forum_view use the same ordering as topic_view) is unreproducible now if you comment out the code.
But it eliminates sorting options while merging topics making it painful finding ones in MCP to merge.

The suggestion is to get rid of unsetting the sorting options there.