I’d suggest that something else is disrupting the setting
It could be a theme or plugin issue
Deactivate all but bbpress and see if this fixes. if it does, re-enable one at a time to see which is causing the error.
If plugins don’t pinpoint the problem, as a test switch to a default theme such as twentyfifteen, and see if this fixes.
Then come back
It took me a ludicrous amount of time to get back to you on this – sorry.
OK, so today I deactivated every plugin except for BB-Press… The forum in question has 45 posts, and they’re all still in a continuous thread. I really can’t fathom why they don’t roll onto a few pages, it must be particuarly irritating for any of my users on mobile.
I then tried with the basic 2017 theme, and that radically changed all the design, but the issue was the same.
Any help would be really gratefully received.
I solved it, well, someone far smarter than I has and I’ve found their answer.
So, I didn’t realise, but you EITHER have nested replies OR thread pages, for BB Press.
With my limited understanding, I’ve simply turned off nested replies (sadly).
But, it does look like it’s possible for people to do both, if they’re a bit more advanced than me!