Is the recount messed up, or does fiora just get magic damage?
Death recap has always had a history of bugginess.
Everything I've heard about it tends to indicate that Riot had to run with some sub-optimal software in exchange for start-up money when the company was founded, so they've had to deal with a mess of code that should be working not working.
Death recap's issues are pretty deep-seeded and require an epic undertaking to attempt ironing out, which Riot has been chipping away at with patch after patch and not a lot to show for it.
Also: Fiora's Riposte does do magic damage last time I checked, but the issue you're running into probably has everything to do with the above reasons.