your screenshots show only PR, but not how the PO looks at scheduling tab in item detail, and if there is already PO item history (beside of showing a proof that the deletion indicator is set)
I did a test in my system and deleted an ordered PO item:
Sorry that it is in German language.
This is the schedule tab from the PO.
If the PR indicator has a checkered flag then it means that the PR is closed and the user gets a warning message 06877 about that fact. If you want reopen the PR while deleting the PO then you should remove this checkered flag.
This is a screenshot from the PR status tab.
As you can see the ordered quantity got reduced to 0 after I had deleted the PO item.
Still the PO can be seen in the status tab.
And looking at the PR quantity tab it can be seen too that the order quantity is zero and the completion indicator is set, corresponding to the checkered flag from the PO
That is the normal way. To evaluate your case, which you already identified as exception to your other PRs, we would need to see some details from the PO. Eventually as well from the PO item change log.