Don't know if this is the case for you, but for me that's what happens when the webpage pops up a modal dialog (often, a "please subscribe to read this article"

Don't know if this is the case for you, but for me that's what happens when the webpage pops up a modal dialog (often, a "please subscribe to read this article") and my adblocker blocks it. The modal gets blocked - but the website also locks down scrolling so you can't just read the article by looking around the modal, and that's what causes the scrollbar to disappear.