I have noticed this strange new behavior a few days ago:
Whenever I open a PDF in the beta reader it’ll load the PDF and a few seconds after loading it and me trying to scroll through the PDF or pulling up the search via CTRL-F it’ll “restart” the pdf reader.
After this “restart” it remains stable.
It behaves like this both in Chrome and Opera (Windows 10)
I’ve noticed it on the latest Chrome 78 and now 79 too, same for Opera.