At the moment soluble support material adhesion is weak due to sparse support layers under soluble support layers. I reported as issue prusa3d#5823 with pictures, as well.
I modified two methods to the SupportMaterial Class including their headers.
The new methods add two base type interface layers to the support structure, in case the extruders are different and soluble support is choosen.
Since it is conditionally activated, it in general doesn't need a GUI input. But a GUI option number of base interface layers may enabled users to adapt this feature to their needs.
This is my second try to provide a pull request on this topic. Reset my fromer repository, because first I merged this and all other changes to my master and couldn't provide separate pull request anymore.