thanks for the feedback and its being looked into, tomorrow i will do some investigations by myself
just to be sure, did you try to bind the controller dpad to your WASD for inventory navigation?
This fixed it for other users so far (via the sub config suggestion i mentioned above, i can post a more detailed "how to" if you want to)
edit:
the concept is the following
right now you are basically using the left analog stick to navigate through the menu
you can also use the controllers dpad to do that through, since Fortnite only analyzes the left controller inputs your dpad will be fully fine when bound to the keyboard
aside from that the controller dpad has the same activation behavior as the keyboard so your inputs will not lock you out of the game
a sub config will take care of your inventory, it will basically switch your WASD to the dpad while you are in the inventory and will switch your WASD back to the character movement when you leave the inventory