This is for packaged builds only (Development). Otherwise works fine in editor.
LoadGameFromSlot does not work when called from a User Widget (UMG) instance. However, you can call the function from another non-UMG blueprint and it works fine.
Workaround: Call LoadGameFromSlot or DoesSaveGameExist from another non-UMG blueprint (for example, from the Game Instance) and return the result to your UMG instance.
This definitely seems like it’s a bug with the engine.
I have attached an image to explain better.