Access control lists at group and space level seems to work intermittently

It seems that if I create a webspace, create a group, populate the group, and give that group space-level edit permission, everything works as advertised. When I then go and add new members to the group, those new members do not get the permissions granted to the rest of the group. I can manually add those new members in the member-level field of the rights object, but they are not "virtually" added based on their membership in the appropriate group.