Not sure what your setup looks like, but soft pointers of any type need to be loaded first. Opening the blueprint in the editor (among other editor actions) load the blueprints, which is why it works after that
Also for your case, if there is no stateful data in your cards, a data asset makes more sense
You can make a new blueprint deriving from PrimaryDataAsset and then make your cards DataAssets of that type instead (miscellaneous → Data Asset)
If the cards are actually instantiated at runtime, by creating new objects, then it’s probably easier to read the data from the instances
Not sure what the packaging error is, if you can post the packaging error message we might be able to help