"Why do my actions in Photoshop disappear and then reappear?"

If I'm to read your question literally, the only thing that can cause that is an intermittent permissions problem in your Windows user account.

Unsaved actions are just stored as a temporay item in your Photoshop Preferences folder.

The preferences are stored in your user account (not in the program files), and are read at application startup. When you close the application, they are rewritten in full. Any irregularities in the startup or shutdown sequences can cause preferences to become corrupt or behave erratically.

Always save out actions, brushes etc and keep them in a safe place.