There's an option checkbox in the in-game settings menu that I believe is called "auto attack"
if it is checked, your champion will auto attack enemies that walk into your attack range without you having to tell them to do so.
if it is not checked then you will only attack enemies by actually commanding your champion to attack them (with targeted attack or the attack-move commands) and will stop attacking after the target is dead or out of visible range
so now, if the checkbox is ON and she's not attacking those minions by herself, then yeah it's likely a bug.
If it's OFF, then not a bug.
If it's ON and she auto attacks normally BEFORE Runaans but stops doing so AFTER buying runaans then probably a bug with runaans item