Yes. before GI from the issuing plant, if GR posted in the receiving plant the stock in transit will display the quantity as negative. You can check the same through MB5T as well. Once the GI posted at issuing plant, it will be balanced and quantity will become zero.
In this case, the negative indiactor indicates that, in-transit there is a no stock and hence it shows the negative sign.. If GI happens first, then MB5T will show the +Ve value. If GR happens first then it will show the negative and once GI posted it will be nullified.
We can automate in such a way that, an out put needs to be triggered from the GI document which will post the GR for the STO.