The problem is that requests are sent for only part of the Nodes in the Controller when above a certain number of Nodes. This certain number is somewhere around 8, this sound a bit vague and it is, but that's the nature of this problem. Sometimes 6 requests are sent, sometimes up to 11. Always more than 6 though.
As part of being a well-behaved netizen QNetworkAccessManager limits the number of simultaneous connections to a single server address. I am fairly sure this limit is 6 connections in the Qt5 source. Are all your connections to the same host?