Looking at the screen recording, I don’t think this is Horizon-related. It appears to be Meta’s default theme.
One detail that may matter is that the video seems to show Chrome’s built-in page translation prompt, translating Meta from English to Simplified Chinese. The Discourse header still appears to show EN.
So this may be more specific to iOS Chrome’s page translation feature interacting with Discourse’s browser navigation and back-button history state.
Could you confirm whether the issue only happens when Chrome’s page translation is enabled?