Sage is currently researching this symptom of quantity on PO becoming out of sync. We are taking all feedback to duplicate the issue. Rebuilding PO sort files will correct the issue in most scenarios. There have been users reporting running the Inventory Reorder report as a commonality.  See kb 64307

We have recently experienced this same issue (May 2021) and have no open Return of Goods entries. We will attempt to correct using BBowers01's suggestion. But I definitely wanted to share this is still happening, and this week is the first time our company has experienced this issue, to the knowledge of current staff.

I believe Beth meant to say to Rebuild Sort Files (not Key Files) and say Yes to the prompt.  You did not mention the version you are using.  There have been a few fixes over the years that were found to be related to this.  Same thing can happen to SO and WO's.  Suggest you Rebuild IM sort files and clear lot/serial committed first.   There are still scenarios where connectivity is lost in the middle of an update or an error occurs stopping the program from completely update all files.  

Well, one of my clients just found this for their issue and let me know I had mis-stated the file.  Yes, it should be Rebuild SORT files.