Role rewards for reaching a certain level are only given to members who are on the specific level of the reward
For example:
When creating a new role reward with level 5 and selecting for it to be retroactively added the role is only given to people exactly on level 5. If you have level 7 you won't receive the role even if there is no specific role for level 7
I have tried both with stacking enabled and disabled and it still only gives the roles to members on the specific level of the award