In the main method of EnsLib.HTTP.OutboundAdapter - SendFormDataArray line 5 you can see the following code:

#; Create an Http Request Object
Set tHttpRequest=$S($$$IsdefObject(pHttpRequestIn):pHttpRequestIn,1:##class(%Net.HttpRequest).%New())  $$$ASSERT($IsObject(tHttpRequest)&&tHttpRequest.%IsA("%Net.HttpRequest"))

Which creates a new empty %Net.HttpRequest object, unless pHttpRequestIn (3rd arg) has been passed with a custom request object. Most wrapper methods (VERB, VERBURL and VERBFormDataArray) do not pass pHttpRequestIn so you should get a fresh request object every time, but SendFormData and SendFormDataURL would pass pHttpRequestIn  from caller, if given.

Another place to look into is a Send call:

Set tSC=tHttpRequest.Send($ZCVT(pOp,"U"),$G(pURL,..URL),..#DEBUG)

It has the following signature:

Method Send(type As %String, location As %String, test As %Integer = 0, reset As %Boolean = 1) As %Status

reset arg defaults to 1, and when it's true the Reset method of %Net.HttpRequest is called after every request. Reset method removes headers (among other things). As you can see in the implementation:

Kill i%Headers,i%FormData,i%Params

EnsLib.HTTP.OutboundAdapter never calls Send with reset=0, so the request should be reset every time.

That said, how do you call SendFormDataArray?

I took a look at the article you linked to. Can you add logging to GetRequest method? It should be called on every business operation message.