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.
This type of mposter command will need to be done every time a propagation policy is edited, until such time as the defect is addressed.
An alternative to executing the mposter command is to modify the pw\server\etc\<cell>\kb\rules\im_internal.mrl file to include this rule:
IM_PROPAGATION_POLICY ($POL) where
[ ($POL.enabled == 1) AND $POL.during_schedule == YES AND $POL.when_slot == '']
$POL.selector_ecf ($EV) where [ $EV.status != CLOSED AND $EV.status != BLACKOUT ]
to all $POL.all_of_propagation_list
to one_of $POL.one_of_propagation_list
recompile the cell with command 'mccomp -n <cell>' and then restart the cell.