Today, a Fiora came in on my tower with a minion wave she used her ultimate it killed me but did not attract the towers agro. She stayed at the tower after I died and at no point did she leave the tower range she was very close to it. Everyone that noticed was pretty surprised. Is this intended? I assume it some sort of bug.
Khalahn is correct. That is the current intended behavior, just as described. The tower can't target Fiora when she's ulting, so she's essentially invisible to the tower. Once your character died, it stopped sending the "distress" call to the tower, so the tower picked a minion. When Fiora became targetable again, the tower had no reason to target her over the minions.