I had asked 7 questions, these 6 are unanswered:
When was your project stock created? what was the movement type used? Has there been any other movement to reduce stock from this specific WBS? Had it successfully reduced stock?
Was there any other sale of project stock from this specific WBS? Did it reduce stock?
Just enter 601Q into the search bar in the SAP Marketplace and you will get to the notes.
But honestly I consider it inconsistent if you can reduce and load stock but cannot issue it with 601Q.
If it had at least issued from normal stock then it could have been something wrong with stock determination. If the inventory was loaded without using the proper programs it could eventually explain why normal movements do not reduce stock. If there was a change in the WBS after the stock was created it could be a reason for this effect. Even certain industry solutions of special features (I still count SIT and CWM as something special as there are some restrictions)
But you have not given any further insight into your processes that could narrow down the options to look at.