So we have come across a weird issue and I am not able to figure out what is causing this. We have some PO's that after they are created and saved, become locked from being able to edit the quantity on the line for any of the items. These are open PO's that are in Pending Receipt status. I have looked at workflows, approval processes, how the PO is created, and there is nothing that would cause this. I thought it might be related to the Rebate SuiteApp that we have installed, but that is not the case, since I was able to disable the scripts and the issue did not resolve. I am at a loss here, any suggestions or thoughts from the experts?

Have you check with script record if you have any another wf/script on PO? Could you closed the line on your PO and re-open and try to edit the quantity Check if you have linked transaction like SO