Shouldn't Kalista not be affected by Tryndamere's slow?

Tryndamere as a champion is perfectly fine; slow, dive, attack, don't die, repeat. It's effective as a kit and against most champions its perfectly fine. However, I feel as though there are circumstances this should not work, in particularly in the case of against a Kalista. Tryndamere's W "Mocking Shout" both lowers Attack Damage. while also slowing those that have their backs to him, therefore if you are facing him then you face no slow. Kalista's passive "Martial Poise" allows her to move while attacking. A good Kalista player can land every auto-attack with a passive movement if they so desire, yet if a Kalista has been hit by a Tryndamere's "Mocking Shout" then she will be slowed, even if she is auto-attacking and moving away with her passive. Since one must be facing a target in order to auto-attack, doesn't that mean that she should not effected by Tryndamere's slow? I believe that this interaction should be changed to allow for a more realistic effect to go along with BOTH of these champions abilities.

                                                                                                                                                                                   Thank You and Sincerely
                                                                                                                                                                                                Hozimina