I was thinking perhaps there might have been something else that was a part of this update that caused the issue - but in fairness I hadn’t upgraded the forums for a while (as I was holding off) so it could be related to one of the other updates.
Shall we split the non-PG posts here into a dedicated Topic to see if we can get to the bottom of it? I’m sure it could prove useful for others too (the guide I wrote for running this kind of set-up is by far my most popular thread here - so it certainly appears that many others are interested in running Discourse on a forum with other sites).