We’ve deployed a fix for the “Back to Main Site” link when clicking on the tiles on the home page. The problem only manifested itself when clicking on the tiles, not when clicking the left-hand links.
The problem was caused by a conflict between the way the tiles were opening the category page and an enhancement to the “Back to Main Site” link in core Discourse. The home page has been modified to use Discourse’s main routing mechanism rather than a simple link, and this resolved the problem.