How to make the radio field checked by default?
I canno't comment to suggest you a solution, but it's probably due to the module field_group (or fieldgroup).

You should try the same content type configuration by using field_collection, because i have never faced any issue with this module.

1/ You create a new field type : field collection. You setup the field with an unlimited number of occurrences.

2/ You go to structure > field collection and add your fields.

I hope it help

Cheers.