I suppose it's a side effect of being a single page application. Mithril doesn't scroll up when you navigate pages, and Flarum didn't implement it either. If you got the code to scroll the page back up (surely there's some simple js or jquery that does that), you can use a system similar to how the Analytics extension tracks page loads, except you run the code to scroll back up.
The tricky part is to exclude discussion pages from that code, otherwise linking to specific posts might get broken.
You could also add the scroll up code to only the init method of the tags page instead of the whole app.