I have had this happen in two scenarios: Via Avatar Saving from Workspace, and ToolBox Clothing assets.
You set the Pants/Shirt ID, save studio and exit; When you return you can observe the shirt/Pants missing from your NPC (The template holder is there; but the asset ID is deleted) Or you copy Pants/Shirts from toolbox items e.g. Uniform givers and when you return the Roblox Asset ID is deleted.
Expected behavior
The asset is saved and not removed.