Steps to reproduce:

I want to have read-write permissions for some specific hosts and read-only for all others.

1. Create user group
2. Add some host groups to it as read-write
3. Save
4. Open it again
5. Add all the hosts as read-only
6. Save
7. Check your user group. 

Result:

You will have read-only permissions for all the host groups

Expected:

Current read-write permissions are not rewritten with the less read-only.