yes, it's creating a new index routeThat addon probably has a setting to change the index route
I've disabled the Friendly URLS and tried to reproduce it, but had no successNo - probably because I don't have friendly URLs enabled and you do. Otherwise, it would've happened if you clicked on Main Category in the breadcrumbs when located on http://demo.extralicense.com/threads/portalpreview.20/ (for example).
I can confirm this too, when simple portal AND route changer are installedBut on my dating forum, it happens clicking onto a category link in breadcrumbs when being on any thread page (i.e., http://datingfora.com/index.php?dating/alpha-males.384/), this page comes up: http://datingfora.com/index.php?forum/#dating-forum.57
The issue disappears when I disable simple portal
That will make it so your portal page is at the index, and the old index page is at "forum".
Note that this addon has a conflict with XenPorta. XenPorta has an option to swap out the index:
But the Route Changer addon overrides this option in XenPorta. That is why I added #2 feature above... to allow you to apply the same change in this addon. So if you use XenPorta and want to keep your portal at the index then you need to disable the above checkbox in XenPorta and then apply the route changes in the above example.