what theme are you using ?
i tried every theme, at the moment the default theme Twenty twenty one, makes no difference
if you have elementor you can try, i wonder why elementor doesn’t detect forum/ as a page
maybe because it is not a real page on a WP basis, forums and topics are, but homepage forum seems not to be (like profiles and search too)
sorry I’m just a bbpress user who helps out here and I have limited time to try stuff.
you could look at
https://codex.bbpress.org/step-by-step-guide-to-setting-up-a-bbpress-forum/ item 3 method 2
Thanks a lot Robin thats exactly what i was looking for 
For those who have the same problem, here is the solution :
Here is how you can do it :
First you need elementor pro so that you can use theme builder
Then on theme builder you add conditions to your template for single post :
include :
Forums > Forums
Forums > Forums by author
and :
Any child of (to include topics too)
Then you exclude :
All pages
All posts
so that the template only applies to the forum
Then you create a page for your forum homepage and you paste : [bbp-forum-index]
on this page you have full control with elementor
the standard homepage is /forums/ so you just need to redirect /forums/ to this page (permanent redirect)
For some reason only profile pages and search pages are not included in this template, but not a real problem in my opinion
Hi
Addition to my previous message :
Best conditions on your forum Elementor template is :
to include forums :
include : Forums > Forums
to include topics :
include : Any child of parent category 1
include : Any child of parent category 2
…
most of the time a forum has 3 to 5 main parent categories, so it’s easy to apply conditions to include all topics
that’s all folks 
Hi Robin, please change in my previous post “category” by “forum” to be more specific
so it gives :
to include topics :
include : Any child of parent forum 1
include : Any child of parent forum 2
…