I think I found the root cause. Delta for CompositeProviders is supported only under specific conditions. This is a restriction on the SAP side, preventing egress.
In delta mode, the DTP supports CompositeProviders (object type HCPR) as sources if they are made up entirely of InfoCubes or of DataStore objects (advanced) that are modeled like standard InfoCubes or standard DataStore objects (classic). The target of the DTP must be a DataStore object (advanced). A combination of InfoCubes and DataStore objects (advanced) as PartProviders of a CompositeProvider is not supported for DTPs in delta mode.
I found a blog detailing workarounds for various Composite Provider limitations. I am unsure whether item number 2 applies to this or not.