Ok ... so working my way slowly through the German 
I take it that you have replaced the standard approval step with a user decision approval step?
Which is fine from a workflow perspective and explains why the workflow works.
The problem is most likely that Approve Leave Requests isn't looking at the workflow but more likely at the leave request status as per the configuration in transaction PTARQ... That would be my guess anyway based on what I've experienced with the wonderful world of leave requests before.
So my suggestion to fix this would be as follows:
- For the Approve Leave Requests app, identify the OData service and method that performs the approve/reject
- Redefine it using the usual OData extension options so that instead of performing the standard approve/reject, it instead updates the work item.
- So you'll need to find the work item related to the leave request (the PTARQ header table holds a reference to the top level workflow id as I recall - and you should be able to find the specific work item from that using the WAPI function modules)
- And then complete the user decision using the appropriate key e.g. by calling SAP_WAPI_DECISION_COMPLETE or similar
- Map the response back to whatever the standard approve/reject expects as a response
In other words, you need to adjust the behaviour of Approve Leave Requests so that it approve/rejects via the workflow instead of with the standard PTARQ approval.
Before you get going though... my question would be whether the custom workflow was changed prior to the Fiori app being introduced or as a result of the Fiori app being introduced?
If your custom workflow already existed before you introduced the Fiori app then that's fine.. you just need to adjust the app behaviour to match.
But if you changed your workflow to try and make the Fiori app behave differently... that may not be the best approach... generally it's better to look at what the extensibility options on the Fiori app give you first and see if there's another alternative. I'm not saying there is definitely a better way as I don't know the Approve Leave Requests app intimately.... just wanting to make sure that the relevant due diligence has been done at your site.