[GAMEPLAY] New Gangplank cannot upgrade his ult.

I was playing the new gangplank, stomping lane. I finally got 500 serpents. I went back to purchase an ult upgrade, When I did, I couldn't buy it. The buy button was just grey. (Like it does when you can't afford something). Even when I reached 1000 serpents, or even 1500 serpents, I still wasn't allowed to buy an upgrade. I have no clue as how to recreate this, and I couldn't get a screenshot, but it did happen

I started by buying a and x3 at the start and went for a as soon as I could. I farmed very well with Parrley vs another Gangplank. I then built after the Occult sword, I made a mistake and bought another but sold it after i had enough to buy . But by then the bug had already occured, So, i'm not sure what could have caused it