When I press Up in command mode in rx 0.4.0 to retrieve a previous command, the cursor remains in its original position at the beginning of the line.

Personally, I see this as the most useless place to put it in this situation but, more objectively, it's at odds with how readline (bash, Python's rlcompleter, etc.), ZLE (zsh), and the Javascript consoles in the Firefox and Chrome developer tools behave. (They put the cursor at the end of the retrieved line, as if you'd just typed it manually.)