On iOS/iPhone, Orion unhides the toolbar almost immediately on scrolling upwards. This is something it has in common with both Safari and Chrome, while Firefox, Firefox Focus, and DuckDuckGo allows some upwards scrolling before unhiding.
As a person who continuously scroll slightly up and down while reading, the Safari/Chrome-model causes sufficient visual noise that I've disabled the hiding and live with the constant presence of toolbars instead.
This is a very preference-dependent suggestion, but hopefully it wouldn't be too difficult to add as an option in the future if there is sufficient interest.