I think there is a bug in Cura... would someone from Ultimaker please confirm this?
While single material extrusion comes out as expected, dual material extrusion (dissolvable support) causes problems with the build material (not support material). It causes the "top surface" layers right underneath the support to be treated as infill instead of a solid area. This problem manifested with both PLA + PVA, and CPE + PLA.
I attached a photograph of the a "Print In Place Gravitational Hook" printed part with the problem that was PLA + PVA, as well as screenshots of the relevant layer previews for the "Dissolvable Support Gearbox" part in dual extrusion mode where it messes up, and in single extrusion mode where it's fine.
Here are also links to YouTube videos I took using the OBS Studio screen capture software that show me scrolling through the layers:
dual extrusion bug in Cura 3.4.1-PPA? (dual extrusion mode layer preview)
dual extrusion bug in Cura 3.4.1-PPA? (single extrusion mode layer preview)
Operating System: Ubuntu 18.04
Software: Cura 3.4.1-PPA
Dissolvable Support Gearbox by tbuser (gear_box.stl)
Print In Place Gravitational Hook by lecter (PrintInPlaceHook.stl)