Sling Weapon resets when using Alter Portal

While Ballistic Ultimate was still running I had a fully charged and upgraded rampage from my Sling equipped. 

I entered an enemy Alter portal. When I exit the portal the game chose to select my 2nd weapon instead of keeping the weapon I had originally equipped.

My expectation would be that I have the same weapon equipped after I exit the portal as I had equipped when I entered the portal.