DataAssets are never loaded in as instances, you only have access to their in-memory representation of the file itself as a CDOs (Class Default Object). Data Assets should be considered Read Only at runtime and not modified.
You might want to make a normal Blueprint Class just extending UObject instead of a DataAsset and create an instance of it at runtime, then you can change it to your heart’s content without it affecting the source asset itself like is happening in your case.