It doesn't seem like anyone else is responding to this issue, so it might be no one else is aware of this HTML encoding issue or knows of a way to fix it.
As such, if you want to get it fixed, then opening a call to SAP is the most likely way to go.
Otherwise, since you mentioned that it's not a problem when a message mapping is used, then you might want to add that into the scenario - not ideal if you were hoping for a passthrough scenario, but there's not much options left from the looks of it.
PS: Stating up front that this is already a live interface and German characters are a new addition would have been very crucial information. This would have prevented some of us trying to force the REST design down your throat! 