Not sure if this issue is still open / followed by anyone. I can reproduce the failure to open PDFs (at the appropriate page) if I specify a PDF reader that is installed as an app from the Microsoft Store (e.g. Drawboard). If instead I specify a PDF reader that is installed as normal software (e.g. PDFXchange) all works fine.
https://forums.zotero.org/discussion/72491/how-to-set-a-windows-store-uwp-app-drawboard-as-custom-pdf-viewer-in-zotero
(This isn't related to the other thread.)