This was observed with filesystem origin, and not yet tested on other origins.

If a decoration file is modified, renamed or moved: The system responds as expected - the ux reflects the change in the definitions. But when the entire definitions directory is removed, or its parent "light module" directory is removed, the system does not "de-register" the decoration.

When system is restarted, the decorations are removed as expected.

To reproduce:
Add a light module to a running instance which includes a decoration.
Observe that the UX shows the decorated definition.
Remove the light module from the instance.
Observe that the UX still shows the decorated definition.

Example attached:
It adds a tab to the mtk textImage.