There was a propagation policy that was working perfectly, until it was edited. After being edited it no longer triggered when expected.
To resolve the situation, the when_slot needs to be mposter'd back in to the cell using:
mposter -n <CELL_NAME> -d -u <DATA_HANDLE#> -b "when_slot=status"
The DATA_HANDLE# can be obtained from the baroc of policy, which itself can be obtained by selecting it in the Admin Console and copy/pasting to a text editor. It can also be found in the cell mcdb file.