Did you find anything out here? or was support able to help you out?
We attempted to re-create this issue on the Beta-Dev Site but found that it worked properly on the beta dev site.
We created new RMA and Receiving PO based on an older Invoice, then tried to return a serialized part and see if we could receive a part we sold back into our inventory, with the correct serial number (without the “-1” )
In the case of the production (live) site, it didn’t work. But in the Dev site, it worked as expected!
Again, I posted info on that thread here: