Is this a bug?

I crafted some time ago a Liberty at 290 and some days ago a Chatterbox with 490. I saw that I can Upgrade the weapons to GS 500 with the blueprint from the vendor, so I farmed the mats and upgraded the Liberty from 290 to 500. But when I wanted to upgrade the Chatterbox it wouldnt let me?

First I thought the weapon needs to be 250 like it says on the blueprint but the Liberty was also above at 290. So why cant I upgrade the Chatterbox?