There is a bug when using the ACF relationship field along with bbPress.
The problem: When an Editor or Author level user loads a page that has a relationship field in it, they are unable to see previously selected values. New selections can be made, and saved. However it will overwrite any previous selections, and after updating the page, it will appear as though there are no selections made again.
Admin level users are able to use relationship fields 100% correctly without any of the above issues.
This issue has actually existing for a long time now, however there was a working fix available in ACF 4.x which no longer works in ACF 5.x.
This support forum topic has great information about the issue, and the fix which worked for 4.x: