In the service B, use clearPipeline as the first step of the service, preserve only the inputs of service B.
In webM IS services, all the variables from the calling service get propagated to the called service (pipeline litter, I remember someone called it), as a best practice, I always use clearPipeline as the first step in the service to preserve just the inputs and get rid of the litter.
HTH, Rohit
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