The reason is here:
https://www.phpbb.com/community/viewtop ... &t=1875595See the 2nd post (from Kevin Clark).
The forum here runs 3.0.10, but upon review of the per-forum settings (there are only 3 forums -- "NES/Famicom", "Other", and "Site Issues"), it was determined that the "Enable active topics" feature was set to No for all of them. I believe this must be the phpBB default when migrating from 2.x to 3.x.
You then have to go into each individual "board" (e.g. for "Site Issues", the "phpBB Issues" board, the "Web Issues" board, etc.) and set the same field to "Yes". I'm in the process of doing that now (will take me some time), and will post a follow-up when finished.