Might this be due to SFDC using the P-R-G technique? P is usually POST but could also be PATCH. It is typically for UIs to avoid users hitting the back button and submitting the same data again but can be useful for APIs too – PATCH is processed, Redirect returned to reference URL affected by the patch, GET to get the latest data which was just changed by the patch.
To confirm that is the behavior, should see the PATCH and a GET in Salesforce log.
Thought this might be something to consider as an alternative explanation.
#webMethods#Integration-Server-and-ESB