When you change build you're still in "select a build" mode. You can press the button for browsing builds to exit that mode - or just Hit 'B' twice to close and re-open the shop. It's not a bug just a bit unclear UI design
Same thing happens here. Sometimes I click on the item and it won't buy it. It might be a bug with buy area detection. UI needs throwing errors when the buy doesn't happen so we know the reason and would be easier to figure future bugs.