When using the EventSource _changes feed support, the browser API will automatically reconnect and send the last sequence it received in the Last-Event-ID header.
The server side needs to use the Last-Event-ID instead of the 'since' query string as a starting point for the changes feed.
You can see the issue by simply creating a database and adding documents and watching what happens to the eventsource _changes feed.
To see the feed with curl use:
curl -i -H 'Last-Event-ID: 2' 'http://localhost:5984/testdb/_changes?feed=eventsource'
the header should also have priority over the since query string:
curl -i -H 'Last-Event-ID: 2' 'http://localhost:5984/testdb/_changes?feed=eventsource?since=1'