Specifically, executing `/services/data/v39.0/actions/standard/createServiceReport` via the Workbench successfully creates the required PDF but making the same call with the same user details from an external REST Client only (but not the same Connected App) only generates an empty PDF file.

Using the OAuth Token as used by the FSL App in the same REST Client does create a correctly populated PDF. It is likely to be the Connected App but I cannot see differences between the two.

I did raise a ticket, and included error codes, but though initially was told that its was symptomatic of a top priority bug, I was later told that only premier partners were eligible for help of the type requested so I don't really know if the call is allowed usage or not.