As you can see on this thread here, we have had reports of this behaviour before. As you can see on the thread, and from what you experienced yourself, it's not so easy to reproduce, and so far, we have been unsuccessful. Until we are able to reproduce this ourselves, there's not much we can do.
The most recent released WinWeb version is 6.4.0 Release c which you can download from our download page here. If you are using the .NETHTML5 version, the most recent version can also be found here.
Release d will be released shortly (don't have dates), and I recall there is one designer fix included in that version. Not sure if it will affect the behaviour you experiences though.
Hope this explains,