To Reproduce:
- Place decal in a scene.
- Enable the "Auto-Repeat" checkbox in the Decal Mesh section of the inspector window.
- Moving the decal correctly triggers the decal mesh to be regenerated.
- Either close the inspector window, or arrange Unity's UI so it's an inactive tab.
- Moving the decal no longer triggers the decal mesh to be regenerated.
Probably because the re-projection is being triggered from UI code that checks for changes rather than being triggered directly by the changes.