Ok, so this is a symptom of a bigger problem. The root problem here is that Discourse should render in desktop view rather than mobile view when the screen is ≥ 925px wide. Can you tell me on what device(s) do you see this problem?
It’s worth mentioning that removing position: sticky may seem to solve this particular problem, but it will break the topic timeline (it will not go down as you scroll down a topic).
Kind of unrelated to this, but I noticed the topic progress bar should be on the left rather than the right when in RTL layout. I sent a PR for this here: FIX: Topic progress bar should be on the left in RTL layouts by OsamaSayegh · Pull Request #11999 · discourse/discourse · GitHub.