Hi Jon,
The task TS20000131 determines its agent via role resolution as per documentation.
Kindly check the documentation:
Standard Task TS20000131 (Approve Trip Facts) - Archiving Application Data (FI) - SAP Library
This may be an issues, as the user you are using may not be a under you as per ORG structure,
in case both are same i.e. INITIATOR & approver it is working!! so need to check how this standard task behaves in custom workflow(which is the case here).
https://help.sap.com/saphelp_46c/helpdata/en/96/aef837aea75351e10000009b38f8cf/content.htm
Consider this as a possibility as you have already exhausted probable common pitfalls.
---BR----
Tapomay