I’ve run into this bug (elb_network_lb not properly supporting subnet_mappings):
https://github.com/ansible/ansible/issues/42977
There is a fix available at:
https://github.com/ansible/ansible/pull/42979
… but I don’t know how to obtain it, install it or use it. I have sent a notification to the chap who seems to be managing it, but in the event that he has no time to answer the queries of a worm such as myself 
I wonder if anybody could give me any pointers?
Thanks, K.