What criteria is necessary for an open requisition to have pre-encumbrance released?

Hello, I have a custom app engine in which I am trying to close requisitions in open and denied status. I am accomplishing this by calling the delivered requisition close process (via CallAppEngine() for PO_REQRCON) and then running a budget check via CreateProcessRequest(). (Budget checking is what releases the pre-encumbrance for closed requisitions.)

The app engine runs normally and does indeed close the requisitions. However, I have noticed that the open requisitions that were closed do not have pre-encumbrances released, whereas the requisitions that were in denied status do have their pre-encumbrances released. (All requisitions that have been closed were definitely picked up by budget checking).