I dont think its issue with wm.EDIINT.rules:processPayload service as it is outofbox builtin service and can you add debug steps (save,restorepipeline) in your custom service that calls the processPayload…
Are you using routeXML or EDIINT:send for processing the 824 outbound via TN?
HTH,
RMG
#Integration-Server-and-ESB#webMethods#edi