Well i would assume it could be because if you changed or added the user role while they were logged in, it might be that their login cookie/session needs to expire or they may need to log out and back in again for the role to apply to that member.
Have you enabled manditory user approval ?
1. Anti-spammer or Watch dog. A must
2. Smart Captcha, and I keep adding questions Monthly.
3. New user roles prevent users from posting till admin approves.
4. CloudFlare Also prevents some spammers, and I can block IP & Countries as needed.
5. User IP tracking so I can check up on new user and make sure they are from where they say they are.
Smart captcha i havent tried, will try now.
I have applied new user roles and it works fine.
I too have cloudfare activated.
And currently i dont have any paid pluggin, so no ip tracking.
As far as the mandatory user approve is the point, i feel forcing someone to wait to get approved is losing popularity.