I didn’t elaborate on the answer as I am unsure of why.
A guess, based on looking at many previews for different parameters:
In case of inadequate print infill overlap on the shells, the slicer compensates with this odd infill to ensure vertical thickness.
I don’t think it is worth overthinking it too much as this must be part of superslicer or prusaslicer… Yet I came from regular use of Cura, so certain things are still new.
BTW, I could eliminate the infill without unselecting the “ensure vertical shell thickness”; you need to play around with the number of walls, infill pattern and density, layer height, line thickness, etc… I was looking for a universal solution to eliminate the extra infill, yet is dependent on multiple variables. Besides additional material and time(?) I am unsure how this influences the print quality.
When I find time, I will try to do some tests to see if the differences are noticeable.