Elastic (the company which hosts and largely staffs these forums) has nothing to do with the AWS Elasticsearch service. AFAIK, they don't support all the APIs that Elasticsearch supports and they seem to have a number of limitations around their service (I think they may cancel reindex commands if you change some cluster settings).
From a format perspective, the curl command you give looks correct. There are some reasons the doc counts may not match, including if some of the docs in index1 don't match the mappings in index2. Logs would indicate this if you could get them.
Anyway, you may want to ask for AWS's support over in the AWS Elasticsearch forums to see if they have any idea what's gone on or you may be interested in using our officially supported Elasticsearch-as-a-service at https://www.elastic.co/cloud.