I can replicate the results that you have but it would gone after a reboot.
To get the internet back again to Rabbits and Puppies is to put an IP address again to Advanced Setup >> LAN accordingly and after pressing save then you will get the internet again. (You may need to use different IP address everytime you are doing this)
In fact I can use 192.168.20.1 for Kittens and 192.168.1.1 for Puppies.
I think there is a bug there on the f/w since logically per Interface Grouping needs to have a valid Layer 2 Interface, so if your atm_0_100 for example has been used by the main group then the other group won't have the internet, unless if you are using 2 connections on the modem at the same time, 1 is for ADSL and 1 is for Fibre for example.
To be honest, I don't think it will be fixed, more likely it's a bug actually since each interface grouping should have a valid layer 2 interface for each one of them to get internet working, so if one already used for the main connection than other interface grouping won't have the internet unless you get another layer 2 interface working such as Fiber for the 2nd connection then you can have a 2nd connection with the 2nd interface grouping.
I have informed this to our engineer and seems they considered it as a bug and if they are going to fix it then you won't have this workaround again in the next f/w release.
But who know if they can do something about it as well.