Had the same problem some time ago, I found that anything you personally do with a WMA file, even a simple move/copy will cause all the info to be lost.
The only way I have found to prevent this is when you are satisfied that you have all the data you need in advanced tag editor, is to select the actual file>properties and make it Read Only.
This prevents access to the Adv tag editor.
If the file is stored in a folder, properties of that folder may show Read Only, do not rely on this, open the folder>Select All>properties>read only.
I'm afraid you'll have to re-enter the info you've lost, but this may help to prevent further disasters.