It has been a long time since I last cast a cylinder but I will try to remember some of the issues. It would be worthwhile to search out my posts on the subject. One thing that comes to mind is that the PLA can be porous which allows the plaster to enter the mold. I solved that by spraying a clear paint onto the mold.
The other thing that comes to mind was that I never got a totally clean pour. The cylinder bore always had tiny pits. Oil retention pits was what I called them. I had decided to just go with an iron liner but got side tracked when I started to build airplanes. *Slightly* more expensive than mopeds. My poor bike sits on the DIY dyno waiting for some love. Maybe I'll get back to it once my second plane is flying.