Stringy base (guesses: Either adhesion issue or too narrow a base)
I did this on an i3 MK3S+:
I've never run into this before, but I'm not sure if that's luck or something else. 😉
The base of this item is 4mm thick. In the past that's always been wide enough for good adhesion. I also wasn't watching it print, so I don't know if it was printing from the outside in or inside out - I'm guessing from the outside in. (I realize you can check each level in PrusaSlicer, but is there any way, with any software, to see how a level is printed, as in where the head starts and the path within the level?)
My guess is it's a bed adhesion issue. I do still have the stick of adhesive that came with my printer kit about a year and a half ago, but I've never needed it. I generally clean my bed with alcohol after each use. Do I have to do anything special, other than rub the end of the adhesive over the bed and clean it carefully with alcohol when the print is done?
RE: Stringy base (guesses: Either adhesion issue or too narrow a base)
In the Prusa slicer, once you have sliced your model, and you go to the preview, you can move the two sliders on the right and bottom to see the print process. The one on the right is for layer height and the one on the bottom is for tool movement.
It may just be a buildup of filament residue on the buld sheet that's not soluble in IPA. Try the dishwashing detergent (Dawn/Fairy) and plenty of HOT water, treatment.