I've marked your answer as correct as I am starting to think the short answer is yes.
It behaves differently from other OBS's.
As I posted
Secondly it is expected that rights based on that OBS to say a group. - In my case that is possible
If OBS based right were given a member of the group sho isuld have eg displayed in the OBS rights of the user properties - in my case they are not
Finally when there are OBS based rights assigned to a user they should work as designed - in my case they don't.
Now that I think back I kinda remember Dave_3.0 pointing out something like that the last two behave just like that.
The final test is to test the rights: do the work as expected. In my case they do.
So to use a financial OBS for access rights the "supported" way is to make the choice before associating that with an entity.
If you think it more cumbersome to change a financial OBS used for access rights than changing the department structure not used for access rights and a copy of it as an OBS structure used for access rights would you please elaborate.