MEMASSPO - blocking/ unblocking PO - inconsistent behaviour

Hi,

I am trying to block [EKPO-LOEKZ = S] a list of PO items using MEMASSPO there seems to be inconsistent behaviour.  It is unblocking the items which are already blocked.  I expect the system to leave the item untouched if it already has EKPO-LOEKZ = S, but it is making this BLANK.  Any idea why this behaviour ?

Below screenshot shows sequence of actions:

Thanks,

Sankar