Hello,
I was unable to reproduce the problem.
I used the plugin you suggested.
1. I created a new role.
2. I went to Users -> (picked a user) -> Capabilities, then assigned the new capability as a secondary capability.
3. I performed an airdrop to that user role. As a test, I used a “Fixed amount airdrop” and I set the “Variable amount airdrop” factor to 0, to keep things simple. The airdrop was applied correctly to the user with the new role only.
The plugin uses the get_users() function to retrieve users with the specified role.
Can you check again? What parameters did you use? Can you verify the above procedure?
with regards