How to reproduce:

1. Install (aws-sdk-plugin and) s3-plugin
2. Open configuration for s3-plugin
3. Create two S3 profiles with different value for "Use IAM Role"
4. Check 'hudson.plugins.s3.S3BucketPublisher.DescriptorImpl'

   public boolean configure(StaplerRequest req, net.sf.json.JSONObject json) throws FormException {
               Logger.getLogger("TEST").log(Level.ALL, "Json" + json.toString());
               Logger.getLogger("TEST").log(Level.ALL, "Params" + req.getParameterMap().toString()); 
   

   Json will contain a inverted values for useRoles instances (true instead false).
   Parameters will contain a value only for first S3Profile.

I replaced optionalBlock to checkbox and it works as expected:
https://github.com/jenkinsci/s3-plugin/commit/a003a69961f07ba11c3494c8946cfffa62232cee