When replying to a post on mobile (when the keyboard is visible), the rest of the topic is hidden behind the reply box. However, when you select an emoji after typing a colon (not from the editor menu bar) it “unpins” the reply box, revealing the posts behind it. This also causes the reply box to jump a bit. I believe this bug is relatively recent, but I can’t say when it started happening.
I am using an iPhone 8, still on the latest 12.x version (not 13) and I’m using Safari.