Let me start by saying that I did extensive searching for past 2 hours and are surprised to see all questions about the same issue getting brushed off. Some even instruct to remove the home link from breadcrumb just to hide an actual bug.
the default Forum Root archive is overriding that page instead other way around. The process should be same as selecting page for posts in WP but in BB is just complicated and not effective.
Use default theme, follow instructions in step 2 , when you done , go to your custom created forum page and change template. No changes. Now change the root in forum settings, redo permalinks go back to custom page and you will see the change.
Note that I have no custom overrides so it is not theme related.