I noticed recently that on mobile (iPhone 8, latest version of iOS 14 at least) sometimes I cannot scroll the page by tapping the page and moving my finger up or down. I finally figured out that this occurs specifically when tapping the topic progress indicator (blue part showing x/y posts) or to its left. It seems to only happen when at the end of the topic, not when the topic progress indicator is “floating” in the middle of the topic. But of course, when it’s fixed at the end of the topic is when it can end up located in the middle of the page and is easiest to tap the section that doesn’t allow scrolling.
Here’s an image showing the section I’m talking about.