So i create a new project, check "Hidden - for creator, group admin and responsible people only" save/ activate project.
Then I realise those not associated to the project can see it.
I check the project status and it's set to "Public - for all logged in users"
I then change it back to "Hidden" and I get an error "There was an error updating this project, please try again." and nothing is changed.
Everything else is working fine.