Service breaks when upgrading from 11.3 to 11.5.1

We have a non-browser based service that works on 11.3 but breaks on 11.5.1. Testing is performed using SoapUI...

Here is the request that is sent to both 11.3 and 11.5.1:

POST  HTTP/1.1
Connection: close
Accept-Encoding: gzip,deflate
Content-Type: text/xml;charset=UTF-8
SOAPAction: ""
Content-Length: 680
Host: x.x.x.x:443
User-Agent: Apache-HttpClient/4.1.1 (java 1.5)

http://schemas.xmlsoap.org/soap/envelope/" xmlns:iden="http://xxxx.com/xxxx" xmlns:rxc="http://xxxx.xxxxx.xxxxx.xxxx.com/xxxx/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

       
      xxxxxxxx
   
   
      
         xxxx
         
            
            

            
            
            
         
      
   

Here is a good 11.3 response:

HTTP/1.1 200 OK
Content-Type: text/xml;charset=UTF-8
Content-Encoding: gzip
Connection: close

http://schemas.xmlsoap.org/soap/envelope/">
;http://xxxx.xxxxxxx.xxxxxx.xxxxx.com/xxx/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" />

Here is a bad 11.5.1 response:

HTTP/1.1 **202** Accepted
Content-Type: text/xml;charset=UTF-8
Content-Length: 0
Connection: close
Set-Cookie:     bbbbbbbbbbbbbbb=FBPHCLGKAACCJNJOBEEPKHOMIJDCALHJHKAKEALGKLNDHFFCBKAMPAJOGAOACCFIACJDCAPEABPIOLKJCOFOMOLHLBKLMNKLKLBFONHDEFKKNCEIDJHKPNMBHDBDOKDF; HttpOnly; secure

I have not modified the string directly above, that's the way it appears in the response. I'm thinking it has something to do with the "gzip" format and how 11.5.1 handles it. Both VIPs are configured identically. HTTP Compression is not enabled, it's a pretty basic VIP config, no special profiles or rules. Has anyone run into this? Any thoughts?