There is a bug with the Magnet Card where if you just make it and throw it in the Magnet Card Slot of the WCT and then try to access it via the Keybind, it isn't referencing the correct magnet Container/Inventory and therefore, the NBT data doesn't get saved. It does however work if you set the magnet info by right-clicking with Magnet Card in-hand and then throw it into the WCT.

So I've spent far too much time on this one little bug. I made so you must first "initialize" the magnet card by right-click with it in-hand before the keybind is able to be used. I didn't want it this way, but this is the only way I could get it to work.