Thanks for your reply mrjarbenne. Posts have started appearing in my activity stream as normal now. I think this issue fixed itself after I ran the “repair forum data” tool from WP admin. I ran that to fix a completely separate issue (bad counts after deleting posts), but it seems to have fixed my issue that stopped forum posts appearing as activity.
There must be a bug in bbPress here, so might be worth the developers looking into if any of them see this. If there’s any more detail I could give to help to that end let me know.