I am currently testing MacOS 26, and I've norticed that since Developer Beta 3 two weeks ago, Skim has stopped working correcty. I can page through a PDF just fine, but when I go back to view previously viewed pages, the app fails to redraw correctly and keeps displaying the last page before going back.
The drawing of the PDF pages is done by Apple's PDFKit, not by our code. Therefore, this is Apple's bug, and you should report it to Apple, not us.