In a MacOS app, I was using a classic PDFView with a transparent background with a simple:
pdfViewer.backgroundColor = .clear
All was working like a charm in Mojave but from the Catalina update I really can fix it. Perhaps it is a bug in relation to Catalyst that could be fixed in the next update but if you see a fix or an alternative it could nice!
I tried to do a pdfViewer.removeFromSuperview() but the windows is well transparent.