I had the same issue today. I queued up auto-buying of several future items, including two vitality items. The first one filled my last free flex slot leaving nowhere for the second one to go. When I walked in front of the shop it popped up the "All slots are full!" UI but the "Sell" and "Cancel" buttons were hidden behind my ability buttons. As such I was softlocked and couldn't close the UI. It didn't even close when I walked away from the shop and I was stuck walking sideways with it for quite a while. I even tried pausing to see if I could get rid of it. Eventually it disappeared after smashing lots of keys but I'm unsure why.
If it helps, I play on a 21:9 ultrawide monitor, which might be why the UI appeared behind other UI? Screenshot attached.